Imaging device and electronic device

ABSTRACT

A highly sensitive imaging device that can perform imaging even under a low illuminance condition is provided. One electrode of a photoelectric conversion element is electrically connected to one of a source electrode and a drain electrode of a first transistor and one of a source electrode and a drain electrode of a third transistor. The other of the source electrode and the drain electrode of the first transistor is electrically connected to a gate electrode of the second transistor. The other electrode of the photoelectric conversion element is electrically connected to a first wiring. A gate electrode of the first transistor is electrically connected to a second wiring. When a potential supplied to the first wiring is HVDD, the highest value of a potential supplied to the second wiring is lower than HVDD.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/007,308, filed Jan. 27, 2016, now pending, which claims the benefitof a foreign priority application filed in Japan as Serial No.2015-016743 on Jan. 30, 2015, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to an imaging deviceincluding an oxide semiconductor.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. One embodiment of thepresent invention relates to a process, a machine, manufacture, or acomposition of matter. Specifically, examples of the technical field ofone embodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a liquid crystaldisplay device, a light-emitting device, a lighting device, a powerstorage device, a storage device, an imaging device, a method fordriving any of them, and a method for manufacturing any of them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a storage device, adisplay device, an imaging device, or an electronic device includes asemiconductor device.

2. Description of the Related Art

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has attractedattention. The transistor is used in a wide range of electronic devicessuch as an integrated circuit (IC) and a display device. A silicon-basedsemiconductor is widely known as a semiconductor material applicable tothe transistor. As another material, an oxide semiconductor hasattracted attention.

For example, a technique for forming a transistor using zinc oxide or anIn—Ga—Zn-based oxide semiconductor as an oxide semiconductor isdisclosed (see Patent Documents 1 and 2).

Patent Document 3 discloses an imaging device in which a transistorincluding an oxide semiconductor and having extremely low off-statecurrent is used in part of a pixel circuit and a transistor includingsilicon with which a complementary metal oxide semiconductor (CMOS)circuit can be formed is used in a peripheral circuit.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2007-123861-   Patent Document 2: Japanese Published Patent Application No.    2007-096055-   Patent Document 3: Japanese Published Patent Application No.    2011-119711

SUMMARY OF THE INVENTION

To obtain a high-resolution image, an image sensor that includes ahighly integrated pixel array is needed. To highly integrate pixels,per-pixel area needs to be reduced.

In the case where the area of a pixel is reduced, the light-receivingarea of a photoelectric conversion element included in the pixel alsoneeds to be reduced. When the light-receiving area of the photoelectricconversion element is reduced, it might be difficult to perform imagingunder a low illuminance condition because of the decrease in sensitivityto light.

An object of one embodiment of the present invention is to provide animaging device that easily performs imaging under a low illuminancecondition. Another object of one embodiment of the present invention isto provide a low-power imaging device. Another object of one embodimentof the present invention is to provide an imaging device that issuitable for high-speed operation. Another object of one embodiment ofthe present invention is to provide an imaging device with highresolution. Another object of one embodiment of the present invention isto provide a highly integrated imaging device. Another object of oneembodiment of the present invention is to provide an imaging device witha wide dynamic range. Another object of one embodiment of the presentinvention is to provide an imaging device that can be used in a widetemperature range. Another object of one embodiment of the presentinvention is to provide an imaging device with a high aperture ratio.Another object of one embodiment of the present invention is to providean imaging device with high reliability. Another object of oneembodiment of the present invention is to provide a novel imaging deviceor the like. Another object of one embodiment of the present inventionis to provide a novel semiconductor device or the like.

The description of these objects does not disturb the existence of otherobjects. In one embodiment of the present invention, there is no need toachieve all the objects. Other objects will be apparent from and can bederived from the description of the specification, the drawings, theclaims, and the like. Other objects will be apparent from and can bederived from the description of the specification, the drawings, theclaims, and the like.

One embodiment of the present invention relates to a highly sensitiveimaging device that can perform imaging even under a low illuminancecondition.

One embodiment of the present invention is an imaging device thatincludes a first transistor, a second transistor, a third transistor, afourth transistor, and a photoelectric conversion element. One electrodeof the photoelectric conversion element is electrically connected to oneof a source electrode and a drain electrode of the first transistor. Theone electrode of the photoelectric conversion element is electricallyconnected to one of a source electrode and a drain electrode of thethird transistor. The other of the source electrode and the drainelectrode of the first transistor is electrically connected to a gateelectrode of the second transistor. One of a source electrode and adrain electrode of the second transistor is electrically connected toone of a source electrode and a drain electrode of the fourthtransistor. The other electrode of the photoelectric conversion elementis electrically connected to a first wiring. A gate electrode of thefirst transistor is electrically connected to a second wiring. When apotential supplied to the first wiring is HVDD, the highest value of apotential supplied to the second wiring is lower than HVDD.

In the above structure, the other of the source electrode and the drainelectrode of the second transistor is electrically connected to a thirdwiring. When the potential supplied to the first wiring is HVDD, apotential supplied to the third wiring is VDD, and HVDD is higher thanVDD, the highest value of the potential supplied to the second wiringcan be VDD.

The first transistor and the third transistor can each include an oxidesemiconductor in an active layer. The oxide semiconductor preferablyincludes In, Zn, and M (M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf).

The second transistor and the fourth transistor can each include siliconin an active layer or an active region.

According to one embodiment of the present invention, an imaging devicethat easily performs imaging under a low illuminance condition can beprovided. A low-power imaging device can be provided. An imaging devicethat is suitable for high-speed operation can be provided. An imagingdevice with high resolution can be provided. A highly integrated imagingdevice can be provided. An imaging device with a wide dynamic range canbe provided. An imaging device that can be used in a wide temperaturerange can be provided. An imaging device with a high aperture ratio canbe provided. An imaging device with high reliability can be provided. Anovel imaging device or the like can be provided. A novel semiconductordevice or the like can be provided.

Note that one embodiment of the present invention is not limited tothese effects. For example, depending on circumstances or conditions,one embodiment of the present invention might produce another effect.Furthermore, depending on circumstances or conditions, one embodiment ofthe present invention might not produce any of the above effects.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates a pixel circuit;

FIG. 2 shows I_(d)-V_(g) characteristics of an OS transistor;

FIG. 3 shows withstand voltage characteristics of an OS transistor;

FIG. 4 shows withstand voltage characteristics of an OS transistor;

FIG. 5 shows withstand voltage characteristics of an OS transistor;

FIG. 6 is a timing chart illustrating operation of imaging;

FIGS. 7A to 7H are circuit diagrams each illustrating a pixel circuit;

FIG. 8 is a circuit diagram illustrating a pixel circuit;

FIGS. 9A to 9C are cross-sectional views each illustrating the structureof an imaging device;

FIGS. 10A and 10B are timing charts illustrating operations of a globalshutter system and a rolling shutter system, respectively;

FIGS. 11A to 11D are cross-sectional views each illustrating connectionof a photoelectric conversion element;

FIGS. 12A and 12B are cross-sectional views each illustrating connectionof a photoelectric conversion element;

FIG. 13 is a cross-sectional view illustrating the structure of animaging device;

FIGS. 14A to 14F are cross-sectional views each illustrating connectionof a photoelectric conversion element;

FIG. 15 is a cross-sectional view illustrating the structure of animaging device;

FIGS. 16A to 16C are cross-sectional views and a circuit diagramillustrating the structure of an imaging device;

FIG. 17 is a cross-sectional view illustrating the structure of animaging device;

FIGS. 18A to 18C are cross-sectional views each illustrating thestructure of an imaging device;

FIG. 19 is a cross-sectional view illustrating the structure of animaging device;

FIG. 20 is a cross-sectional view illustrating the structure of animaging device;

FIG. 21 is a cross-sectional view illustrating the structure of animaging device;

FIG. 22 is a cross-sectional view illustrating the structure of animaging device;

FIGS. 23A1, 23A2, 23A3, 23B1, 23B2, and 23B3 illustrate a bent imagingdevice;

FIGS. 24A to 24F are top views and cross-sectional views illustrating atransistor;

FIGS. 25A to 25F are top views and cross-sectional views illustrating atransistor;

FIGS. 26A to 26D each illustrate a cross section of a transistor in achannel width direction;

FIGS. 27A to 27F each illustrate a cross section of a transistor in achannel length direction;

FIGS. 28A to 28E are a top view and cross-sectional views illustrating asemiconductor layer;

FIGS. 29A to 29F are top views and cross-sectional views illustrating atransistor;

FIGS. 30A to 30F are top views and cross-sectional views illustrating atransistor;

FIGS. 31A to 31D each illustrate a cross section of a transistor in achannel width direction;

FIGS. 32A to 32F each illustrate a cross section of a transistor in achannel length direction;

FIGS. 33A to 33C are top views each illustrating a transistor;

FIGS. 34A to 34F illustrate electronic devices;

FIG. 35 illustrates a pixel circuit;

FIG. 36 illustrates a pixel circuit;

FIGS. 37A and 37B each illustrate a pixel circuit;

FIG. 38 illustrates a pixel circuit.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription. It will be readily appreciated by those skilled in the artthat modes and details of the present invention can be modified invarious ways without departing from the spirit and scope of the presentinvention. The present invention therefore should not be construed asbeing limited to the following description of the embodiments. Instructures of the invention described below, the same portions orportions having similar functions are denoted by the same referencenumerals in different drawings, and the description thereof is notrepeated in some cases. The same components are denoted by differenthatching patterns in different drawings, or the hatching patterns areomitted in some cases.

For example, in this specification and the like, an explicit description“X and Y are connected” means that X and Y are electrically connected, Xand Y are functionally connected, and X and Y are directly connected.Accordingly, without being limited to a predetermined connectionrelationship, for example, a connection relationship shown in drawingsor texts, another connection relationship is included in the drawings orthe texts.

Here, each of X and Y denotes an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, or alayer).

Examples of the case where X and Y are directly connected include thecase where an element that enables electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) is notconnected between X and Y, and the case where X and Y are connectedwithout the element that enables electrical connection between X and Yprovided therebetween.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. Note that the switch is controlled to beturned on or off. That is, the switch is conducting or not conducting(is turned on or off) to determine whether current flows therethrough ornot. Alternatively, the switch has a function of selecting and changinga current path. Note that the case where X and Y are electricallyconnected includes the case where X and Y are directly connected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a D/A converter circuit, anA/D converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; astorage circuit; or a control circuit) can be connected between X and Y.Note that for example, in the case where a signal output from X istransmitted to Y even when another circuit is provided between X and Y,X and Y are functionally connected. The case where X and Y arefunctionally connected includes the case where X and Y are directlyconnected and X and Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or another circuit provided therebetween), X and Y arefunctionally connected (i.e., the case where X and Y are functionallyconnected with another circuit provided therebetween), and X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween). That is, inthis specification and the like, the explicit description “X and Y areelectrically connected” is the same as the explicit description “X and Yare connected.”

For example, the case where a source (or a first terminal or the like)of a transistor is electrically connected to X through (or not through)Z1 and a drain (or a second terminal or the like) of the transistor iselectrically connected to Y through (or not through) Z2, or the casewhere a source (or a first terminal or the like) of a transistor isdirectly connected to part of Z1 and another part of Z1 is directlyconnected to X while a drain (or a second terminal or the like) of thetransistor is directly connected to part of Z2 and another part of Z2 isdirectly connected to Y, can be expressed by using any of the followingexpressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in that order,” “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin that order,” and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are connected in that order.” When theconnection order in a circuit structure is defined by an expressionsimilar to the above examples, a source (or a first terminal or thelike) and a drain (or a second terminal or the like) of a transistor canbe distinguished from each other to specify the technical scope.

Other examples of the expressions include “a source (or a first terminalor the like) of a transistor is electrically connected to X through atleast a first connection path, the first connection path does notinclude a second connection path, the second connection path is a pathbetween the source (or the first terminal or the like) of the transistorand a drain (or a second terminal or the like) of the transistor, Z1 ison the first connection path, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leasta third connection path, the third connection path does not include thesecond connection path, and Z2 is on the third connection path.” It isalso possible to use the expression “a source (or a first terminal orthe like) of a transistor is electrically connected to X through atleast Z1 on a first connection path, the first connection path does notinclude a second connection path, the second connection path includes aconnection path through the transistor, a drain (or a second terminal orthe like) of the transistor is electrically connected to Y through atleast Z2 on a third connection path, and the third connection path doesnot include the second connection path.” Still another example of theexpressions is “a source (or a first terminal or the like) of atransistor is electrically connected to X through at least Z1 on a firstelectrical path, the first electrical path does not include a secondelectrical path, the second electrical path is an electrical path fromthe source (or the first terminal or the like) of the transistor to adrain (or a second terminal or the like) of the transistor, the drain(or the second terminal or the like) of the transistor is electricallyconnected to Y through at least Z2 on a third electrical path, the thirdelectrical path does not include a fourth electrical path, and thefourth electrical path is an electrical path from the drain (or thesecond terminal or the like) of the transistor to the source (or thefirst terminal or the like) of the transistor.” When the connection pathin a circuit structure is defined by an expression similar to the aboveexamples, a source (or a first terminal or the like) and a drain (or asecond terminal or the like) of a transistor can be distinguished fromeach other to specify the technical scope.

Note that these expressions are examples and there is no limitation onthe expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, the term “electrical connection” in thisspecification also means such a case where one conductive film hasfunctions of a plurality of components.

Note that the terms “film” and “layer” can be interchanged with eachother depending on circumstances or conditions. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. In addition, the term “insulating film” can be changed intothe term “insulating layer” in some cases.

Note that in general, a potential (voltage) is relative and isdetermined depending on the amount relative to a certain potential.Therefore, even when the expression “ground”, “GND”, or the like isused, the potential is not necessarily 0 V. For example, the “groundpotential” or “GND” might be defined using the lowest potential in acircuit as a reference. Alternatively, the “ground potential” or “GND”might be defined using an intermediate potential in a circuit as areference. In those cases, a positive potential and a negative potentialare set using the potential as a reference.

Embodiment 1

In this embodiment, an imaging device that is one embodiment of thepresent invention is described with reference to drawings.

FIG. 1 illustrates a pixel circuit that can be used for an imagingdevice in one embodiment of the present invention. The pixel circuitincludes a transistor 51, a transistor 52, a transistor 53, a transistor54, and a photoelectric conversion element 60.

One electrode of the photoelectric conversion element 60 is electricallyconnected to one of a source electrode and a drain electrode of thetransistor 51. The one electrode of the photoelectric conversion element60 is electrically connected to one of a source electrode and a drainelectrode of the transistor 53. The other of the source electrode andthe drain electrode of the transistor 51 is electrically connected to agate electrode of the transistor 52. One of a source electrode and adrain electrode of the transistor 52 is electrically connected to one ofa source electrode and a drain electrode of the transistor 54.

The other electrode of the photoelectric conversion element 60 iselectrically connected to a wiring 72 (HVDD). A gate electrode of thetransistor 51 is electrically connected to a wiring 75 (TX). The otherof the source electrode and the drain electrode of the transistor 52 iselectrically connected to a wiring 79 (VDD). A gate electrode of thetransistor 53 is electrically connected to a wiring 76 (RS). The otherof the source electrode and the drain electrode of the transistor 53 iselectrically connected to a wiring 73 (GND). The other of the sourceelectrode and the drain electrode of the transistor 54 is electricallyconnected to a wiring 71 (OUT). A gate electrode of the transistor 54 iselectrically connected to a wiring 78 (SE). The wiring 72 (HVDD) iselectrically connected to one terminal of a high voltage power source56. The other terminal of the high voltage power source 56 iselectrically connected to a wiring 77 (GND).

The wiring 71 (OUT) can function as an output line that outputs a signalfrom a pixel. The wiring 73 (GND), the wiring 77 (GND), and the wiring79 (VDD) can function as power supply lines. For example, the wiring 73(GND) and the wiring 77 (GND) can function as low potential power supplylines, and the wiring 79 (VDD) can function as a high potential powersupply line. The wiring 75 (TX), the wiring 76 (RS), and the wiring 78(SE) can function as signal lines that control the on/off states of thetransistors.

Note that the wiring 73 (GND) and the wiring 77 (GND) may be provided asone wiring. In addition, the potentials of the two wirings are notlimited to GND, and may be any potentials as long as they aresufficiently lower than a potential supplied to the wiring 79 (VDD).

The photoelectric conversion element 60 exhibits photoelectriccharacteristics when a potential HVDD that is high voltage is applied.Note that in this embodiment, the potential HVDD is higher than apotential VDD supplied to the wiring 79 (VDD). To increase lightdetection sensitivity under a low illuminance condition, it ispreferable to use a photoelectric conversion element formed using amaterial that causes avalanche charge multiplication as thephotoelectric conversion element 60. To cause avalanche chargemultiplication, comparatively high voltage (HVDD) is needed. Thus, thehigh voltage power source 56 is capable of supplying HVDD, and HVDD issupplied to the other electrode of the photoelectric conversion element60 through the wiring 72 (HVDD).

The transistor 51 can function as a transfer transistor for transferringthe potential of a charge accumulation portion (NR) that changes inresponse to output of the photoelectric conversion element 60 to acharge accumulation portion (ND). The transistor 53 can function as areset transistor that initializes the potentials of the chargeaccumulation portion (NR) and the charge accumulation portion (ND). Thetransistor 52 can function as an amplifying transistor that outputs asignal based on the potential of the charge accumulation portion (ND).The transistor 54 can function as a selection transistor that selects apixel from which a signal is read.

In the case where high voltage is applied to the photoelectricconversion element 60, a transistor to be connected to the photoelectricconversion element 60 needs to withstand the high voltage. As the highwithstand voltage transistor, for example, a transistor including anoxide semiconductor in an active layer (hereinafter referred to as an OStransistor) can be used. Specifically, OS transistors are preferablyused as the transistors 51 and 53.

Here, FIG. 2 shows static characteristics (drain voltage (V_(d))=0.1 Vand 3.3 V) of an OS transistor with channel length L/channel widthW=0.35 μm/0.8 μm and a gate insulating film thickness (obtained byconversion) of 20 nm (a physical film thickness of 31 nm). FIG. 3 showsdrain breakdown voltage characteristics of the transistor at gatevoltage (V_(g))=0 V and source voltage (V_(s))=0 V. FIG. 4 showspositive gate breakdown voltage characteristics of the transistor atV_(s)=V_(d)=0 V. FIG. 5 shows negative gate breakdown voltagecharacteristics of the transistor at V_(s)=V_(d)=0 V. As shown in FIG.2, FIG. 3, FIG. 4, and FIG. 5, the OS transistor has high withstandvoltage electrical characteristics.

Since the OS transistor has a wide bandgap (>3.0 eV) semiconductor,drain withstand voltage depends not on junction withstand voltage but onthe gate insulating film thickness. The OS transistor is less likely togenerate a short channel effect, so that the drain withstand voltage isincreased by making the gate insulating film thicker and normally-offtransistor characteristics are easily obtained. The OS transistorwithstands a bias of higher than 20 V that is necessary for avalanchecharge multiplication of the photoelectric conversion element. Note thatthe failure mode of the OS transistor is dielectric breakdown.

Note that in this specification, the high withstand voltage transistorrefers to a transistor that can withstand desired high voltage withoutelectrical breakdown. For example, in the high withstand voltagetransistor, desired high voltage can be applied to a gate electrodewithout breakdown when GND is applied to a source electrode and a drainelectrode. Alternatively, in the high withstand voltage transistor,desired high voltage can be applied to the drain electrode withoutbreakdown when GND is applied to the source electrode and the gateelectrode. Alternatively, in the high withstand voltage transistor,desired high voltage can be applied to the drain electrode withoutbreakdown when GND is applied to the source electrode and VDD is appliedto the gate electrode.

Although the transistors 51 and 53 give high priority to switchingcharacteristics, the transistor 52 gives high priority to amplifyingcharacteristics; thus, a transistor with high on-state current ispreferably used. Therefore, a transistor including silicon in an activelayer or an active region (hereinafter referred to as a Si transistor)is preferably used as the transistor 52.

Note that the highest potential of the charge accumulation portion (NR)can be HVDD. However, in the case where an n-channel transistor is usedas the transistor 51, the potential of the charge accumulation portion(ND) is not higher than the highest potential that is applied to thegate electrode of the transistor 51. More properly, the potential of thecharge accumulation portion (ND) is not higher than a potential obtainedby subtracting the threshold voltage (V_(th)) of the transistor 51 fromthe highest potential that is applied to the gate electrode of thetransistor 51. For example, even if the potential of the chargeaccumulation portion (NR) is HVDD, the highest potential of the chargeaccumulation portion (ND) is VDD when the highest potential that isapplied to the wiring 75 (TX) is VDD. More properly, the potential ofthe charge accumulation portion (ND) is VDD−V_(th). Note that in thefollowing description, V_(th) of the transistor 51 is regarded as lowvoltage, and the description of V_(th) of the transistor 51 is omittedwhen the potential of the charge accumulation portion (ND) is described.

In other words, when the transistor 51 is normally-off, the potentialVDD that is lower than the potential HVDD is power supply voltage forreading. Therefore, high voltage is not applied to the gate electrode ofthe transistor 52, so that the use of a Si transistor that includes athin gate insulating film and has comparatively not so high withstandvoltage as the transistor 52 does not pose a problem.

When the transistors 51 to 54 have the above structures, it is possibleto manufacture an imaging device that has high light detectionsensitivity under a low illuminance condition and can output a signalwith little noise. Since the imaging device has high light detectionsensitivity, light capturing time can be shortened and imaging can beperformed at high speed.

Next, operation of the pixel is described with reference to a timingchart in FIG. 6. Note that in the following description of an operationexample, HVDD and GND are supplied to the wiring 76 (RS) connected tothe gate electrode of the transistor 53 as an “H” potential and an “L”potential, respectively. In addition, VDD and GND are supplied to thewiring 75 (TX) connected to the gate electrode of the transistor 51 asan “H” potential and an “L” potential, respectively. Furthermore, VDD issupplied to the wiring 79 (VDD) connected to the source electrode of thetransistor 52. Other potentials also can be supplied to the wirings.

First, if the wiring 76 (RS) is set at “H” and the wiring 75 (TX) is setat “H,” the potentials of the charge accumulation portion (NR) and thecharge accumulation portion (ND) are each set to a reset potential (GND)(that is, reset operation). Note that in reset operation, VDD may besupplied to the wiring 76 (RS) as an “H” potential.

Next, if the wiring 76 (RS) is set at “L” and the wiring 75 (TX) is setat “L,” the potential of the charge accumulation portion (NR) changes(that is, accumulation operation). The potential of the chargeaccumulation portion (NR) is changed from GND to HVDD at the maximumdepending on the intensity of light that enters the photoelectricconversion element 60.

If the wiring 75 (TX) is set at “H” subsequent to the accumulationoperation, charge in the charge accumulation portion (NR) is transferredto the charge accumulation portion (ND) (that is, transfer operation).

Although the potential of the charge accumulation portion (NR) ischanged depending on the intensity of light that enters thephotoelectric conversion element 60 in the accumulation operation, VDDis supplied to the gate electrode of the transistor 51; thus, when thepotential of the charge accumulation portion (ND) reaches VDD, thetransistor 51 is turned off. Therefore, the potential of the chargeaccumulation portion (ND) is changed from the reset potential (GND) toVDD at the maximum. In other words, VDD is applied to the gate electrodeof the transistor 52 at the maximum.

Note that although the wiring 75 (TX) is set at “L” in the accumulationoperation in FIG. 6, the wiring 75 (TX) may be set at “H.” In that case,the potential of the charge accumulation portion (ND) is changed inaccordance with the potential change of the charge accumulation portion(NR); however, VDD is supplied to the gate electrode of the transistor51. Therefore, when the potential of the charge accumulation portion(ND) reaches VDD, the transistor 51 is turned off. Thus, the potentialof the charge accumulation portion (ND) is changed from the resetpotential (GND) to VDD at the maximum. In other words, also in such acase, VDD is applied to the gate electrode of the transistor 52 at themaximum.

Note that if the wiring 75 (TX) is set at “L” in the accumulationoperation, the influence of noise caused by the transistor 51 can bereduced. In contrast, if the wiring 75 (TX) is set at “H,” the influenceof noise caused by switching of the transistor 51 can be reduced.

If the wiring 76 (RS) is set at “L,” the wiring 75 (TX) is set at “L,”and the wiring 78 (SE) is set at “H” subsequent to the transferoperation, a signal based on the potential of the charge accumulationportion (ND) can be output to the wiring 71 (OUT). In other words, anoutput signal based on the intensity of light that enters thephotoelectric conversion element 60 in the accumulation operation can beobtained.

In the above operation, the highest voltages applied to the terminals ofthe transistors are as follows. In some cases, HVDD is applied to thesource electrode of the transistor 51, and VDD is applied to the gateelectrode and the drain electrode of the transistor 51. In some cases,HVDD is applied to the gate electrode and the drain electrode of thetransistor 53, and GND is applied to the source electrode of thetransistor 53. In some cases, VDD is applied to the source electrode,the drain electrode, and the gate electrode of the transistor 52. Insome cases, VDD is applied to the source electrode, the drain electrode,and the gate electrode of the transistor 54. Therefore, the transistors51 and 53 need to withstand the high voltage HVDD. In contrast, it isenough for the transistors 52 and 54 to withstand VDD.

Note that the pixel circuit used for the imaging device in oneembodiment of the present invention may have a structure in FIG. 35. Inthe pixel circuit in FIG. 35, one of a source electrode and a drainelectrode of a transistor 55 is connected to the charge accumulationportion (ND) in FIG. 1. The other of the source electrode and the drainelectrode of the transistor 55 is connected to a wiring 93 (GND). A gateelectrode of the transistor 55 is connected to a wiring 96 (RS). Thewiring 93 (GND) can have the same potential as the wiring 73 (GND). Thewiring 96 (RS) is a signal line for controlling the transistor 55 andcan be supplied with a potential similar to that of the wiring 76 (RS).The wiring 76 (RS) and the wiring 96 (RS) can be supplied with differentpotentials. Furthermore, the wiring 76 (RS) and the wiring 96 (RS) canbe electrically connected to each other.

In the pixel circuit structure in FIG. 1, the reset operation of thecharge accumulation portion (ND) is performed by turning on thetransistors 51 and 53. However, in the structure in FIG. 35, the chargeaccumulation portion (ND) can be reset without operation of thetransistor 51; thus, imaging noise can be reduced.

In the circuit structure in FIG. 1, the transistor 54 is providedbetween the transistor 52 and the wiring 71 (OUT). However, asillustrated in FIG. 36, the transistor 54 may be provided between thetransistor 52 and the wiring 79 (VDD).

In the circuit structure in FIG. 1, the other of the source electrodeand the drain electrode of the transistor 52 is connected to the highpotential power supply line (the wiring 79 (VDD)). However, asillustrated in FIG. 37A, the other of the source electrode and the drainelectrode of the transistor 52 may be connected to a low potential powersupply line (a wiring 99 (GND)). As illustrated in FIG. 37B, thetransistor 52 may be replaced with a p-channel transistor.

As another structure of the pixel circuit in one embodiment of thepresent invention, the transistor 54 may be omitted as illustrated inFIG. 7A. As illustrated in FIG. 7B, a capacitor 57 may be provided forthe charge accumulation portion (NR). As illustrated in FIG. 7C, acapacitor 58 may be provided for the charge accumulation portion (ND).As illustrated in FIG. 7D, the capacitors 57 and 58 may be provided.Note that any of the structures in FIGS. 7A to 7D can be optionallycombined with any of the structures in FIG. 35, FIG. 36, and FIGS. 37Aand 37B.

The transistors 51 and 53 in the pixel circuit may each include a backgate as illustrated in FIGS. 7E and 7F. FIG. 7E illustrates a structurein which a constant potential is applied to the back gates, whichenables control of the threshold voltage. FIG. 7F illustrates astructure in which the back gates are supplied with the same potentialas front gates, which enables an increase in on-state current. Thetransistors 51 to 54 may each include a back gate as illustrated inFIGS. 7G and 7H. The circuits in FIG. 1, FIGS. 7A to 7D, FIG. 35, FIG.36, FIGS. 37A and 37B can also have transistors having back gates.Moreover, a structure in which the same potential is applied to a frontgate and a back gate, a structure in which a constant potential isapplied to a back gate, and a structure without a back gate may beoptionally combined as necessary for the transistors included in onepixel circuit.

Note that the pixel circuit may have a structure in which thetransistors 52 and 54 are shared among a plurality of pixels asillustrated in FIG. 8. FIG. 8 illustrates a structure in which thetransistors 52 and 54 are shared among a plurality of pixels in aperpendicular direction; however, the transistors 52 and 54 may beshared among a plurality of pixels in a horizontal direction or in ahorizontal and perpendicular direction. Such a structure can reduce thenumber of transistors included in one pixel. As illustrated in FIG. 38,the charge accumulation portion (ND) may have the reset transistor 55.

Although FIG. 8 and FIG. 38 each illustrate a structure in which thetransistors 52 and 54 are shared among four pixels, the transistors 52and 54 may be shared among two pixels, three pixels, or five or morepixels. Note that the structure in FIG. 8 or FIG. 38 can be optionallycombined with any of the structures in FIGS. 7A to 7H, FIG. 36, andFIGS. 37A and 37B.

Such a structure can provide an imaging device that includes a highlyintegrated pixel array. Alternatively, an imaging device that easilyperforms imaging under a low illuminance condition can be provided.

Next, specific structure examples of the imaging device in oneembodiment of the present invention are described below with referenceto drawings. FIG. 9A is an example of a cross-sectional view of theimaging device in one embodiment of the present invention andillustrates an example of specific connection between the photoelectricconversion element 60 and the transistors 51 and 53 which are includedin the pixel in FIG. 1. Note that FIG. 9A does not illustrate thetransistors 52 and 54. The imaging device includes a layer 1100including the transistors 51 to 54 and a layer 1200 including thephotoelectric conversion element 60.

Although the wirings, the electrodes, and conductors 81 are illustratedas independent components in cross-sectional views in this embodiment,some of them are provided as one component in some cases when they areelectrically connected to each other. In addition, a structure in whicha gate electrode, a source electrode, or a drain electrode of thetransistor is connected to the wirings through the conductor 81 is onlyan example, and the gate electrode, the source electrode, and the drainelectrode of the transistor might each function as a wiring.

In addition, insulating layers 82 and 83 and the like that function asprotective films, interlayer insulating films, or planarization filmsare provided over the components. For example, an inorganic insulatingfilm such as a silicon oxide film or a silicon oxynitride film can beused as each of the insulating layers 82 and 83 and the like.Alternatively, an organic insulating film such as an acrylic resin filmor a polyimide resin film may be used. Top surfaces of the insulatinglayers 82 and 83 and the like are preferably planarized by chemicalmechanical polishing (CMP) or the like as necessary.

In some cases, one or more of the wirings and the like illustrated inthe drawing are not provided or a wiring, a transistor, or the like thatis not illustrated in the drawing is included in each layer. Inaddition, a layer that is not illustrated in the drawing might beincluded. Furthermore, one or more of the layers illustrated in thedrawing are not included in some cases.

It is particularly preferable to use OS transistors as the transistors51 and 53.

Extremely low off-state current of the OS transistor can widen thedynamic range of imaging. In the circuit structure of the pixelillustrated in FIG. 1, a decrease in the intensity of light entering thephotoelectric conversion element 60 reduces the potential of the chargeaccumulation portion (ND). Since the OS transistor has extremely lowoff-state current, current based on a gate potential can be accuratelyoutput even when the gate potential is extremely low. Thus, it ispossible to widen the detection range of illuminance, i.e., the dynamicrange.

A period during which charge can be held in the charge accumulationportion (ND) and the charge accumulation portion (NR) can be extremelylong owing to the low off-state current of the transistors 51 and 53.Therefore, a global shutter system in which accumulation operation isperformed in all the pixels at the same time can be used without acomplicated circuit structure and operation method.

In general, in an imaging device where pixels are arranged in a matrix,a rolling shutter system is employed in which imaging operation 11,retention operation 12, and read operation 13 are performed row by rowas illustrated in FIG. 10A. In the case of employing the rolling shuttersystem, simultaneousness of imaging is lost. Therefore, when an objectmoves, an image is distorted.

As a result, in one embodiment of the present invention, it ispreferable to employ a global shutter system in which the imagingoperation 11 can be performed simultaneously in all the rows and theread operation 13 can be sequentially performed row by row asillustrated in FIG. 10B. By employing the global shutter system,simultaneousness of imaging in all the pixels in the imaging device canbe secured, and an image with little distortion can be easily formedeven when an object moves.

In addition, the OS transistor has lower temperature dependence ofchange in electrical characteristics than a Si transistor, and thus canbe used in an extremely wide range of temperatures. Therefore, animaging device and a semiconductor device that include the OStransistors are suitable for automobiles, aircrafts, and spacecrafts.

Moreover, the OS transistor has higher drain breakdown voltage than theSi transistor. In a photoelectric conversion element in which aselenium-based material is used for a photoelectric conversion layer,comparatively high voltage (e.g., 10 V or higher) is preferably appliedto cause an avalanche phenomenon easily. Therefore, by combination ofthe OS transistor and the photoelectric conversion element in which theselenium-based material is used for the photoelectric conversion layer,a highly reliable imaging device can be obtained.

Note that although each transistor includes a back gate in FIG. 9A, asillustrated in FIG. 9B, each transistor does not necessarily include aback gate. Alternatively, as illustrated in FIG. 9C, one or moretransistors, for example, only the transistor 51 may include a backgate. The back gate might be electrically connected to a correspondingfront gate of the transistor. Alternatively, different fixed potentialsmight be supplied to the back gate and the front gate. Note that thepresence or absence of the back gate can also be applied to anotherimaging device described in this embodiment.

A variety of elements can be used as the photoelectric conversionelement 60 provided in the layer 1200. FIG. 9A illustrates thephotoelectric conversion element 60 including a selenium-based materialfor a photoelectric conversion layer 61. The photoelectric conversionelement 60 including a selenium-based material has high external quantumefficiency with respect to visible light. In the photoelectricconversion element, a highly sensitive sensor in which the amount ofamplification of electrons with respect to the amount of incident lightby an avalanche phenomenon is large can be obtained. Furthermore, theselenium-based material has a high light absorption coefficient, whichleads to an advantage that the photoelectric conversion layer 61 iseasily formed to be thin.

Amorphous selenium or crystalline selenium can be used as aselenium-based material. Crystalline selenium can be obtained by, forexample, depositing amorphous selenium and then performing heattreatment. When the crystal grain size of crystalline selenium issmaller than a pixel pitch, variation in characteristics between pixelscan be reduced. Moreover, crystalline selenium has higher spectralsensitivity and light-absorption coefficient than those of amorphousselenium.

Furthermore, the photoelectric conversion layer 61 may be a layerincluding a compound of copper, indium, and selenium (CIS).Alternatively, a layer including a compound of copper, indium, gallium,and selenium (CIGS) may be used. With the CIS layer or the CIGS layer, aphotoelectric conversion element that can utilize an avalanchephenomenon in a manner similar to that of a single layer of selenium canbe formed.

In the photoelectric conversion element 60 including a selenium-basedmaterial, for example, the photoelectric conversion layer 61 can beprovided between a light-transmitting conductive layer 62 and theelectrode 66 formed using a metal material or the like. CIS and CIGS arep-type semiconductors and may be formed in contact with an n-typesemiconductor such as cadmium sulfide or zinc sulfide to form ajunction.

Comparatively high voltage (e.g., 10 V or higher) is preferably appliedto the photoelectric conversion element to cause the avalanchephenomenon. Since the OS transistor has higher drain breakdown voltagethan the Si transistor, comparatively high voltage can be easily appliedto the photoelectric conversion element. Therefore, by combination ofthe OS transistor with high drain breakdown voltage and thephotoelectric conversion element in which the selenium-based material isused for the photoelectric conversion layer, a highly sensitive, highlyreliable imaging device can be obtained.

Although the photoelectric conversion layer 61 and thelight-transmitting conductive layer 62 are not divided between pixelcircuits in FIG. 9A, the photoelectric conversion layer 61 and thelight-transmitting conductive layer 62 may be divided between circuitsas illustrated in FIG. 11A. Furthermore, a partition wall 67 formedusing an insulator is preferably provided in a region between pixelswhere the electrode 66 is not provided so as not to generate a crack inthe photoelectric conversion layer 61 and the light-transmittingconductive layer 62; however, the partition wall 67 is not necessarilyprovided as illustrated in FIG. 11B. Although the light-transmittingconductive layer 62 and the wiring 72 are connected to each otherthrough a wiring 88 and the conductor 81 in FIG. 9A, thelight-transmitting conductive layer 62 and the wiring 72 may be indirect contact with each other as in FIGS. 11C and 11D.

The electrode 66, the wiring 72, and the like may each be a multilayer.For example, as illustrated in FIG. 12A, the electrode 66 can includetwo conductive layers 66 a and 66 b and the wiring 72 can include twoconductive layers 72 a and 72 b. In the structure of FIG. 12A, forexample, the conductive layers 66 a and 72 a may be made of alow-resistance metal or the like, and the conductive layers 66 b and 72b may be made of a metal or the like that exhibits an excellent contactproperty with the photoelectric conversion layer 61. Such a structureimproves the electrical properties of the photoelectric conversionelement. Furthermore, even when the conductive layer 72 a contains ametal that causes electrolytic corrosion, which occurs when some kindsof metal are in contact with the light-transmitting conductive layer 62,electrolytic corrosion can be prevented because the conductive layer 72b is placed between the conductive layer 72 a and the light-transmittingconductive layer 62.

The conductive layers 66 b and 72 b can be formed using, for example,molybdenum, tungsten, or the like. The conductive layers 66 a and 72 acan be formed using, for example, aluminum, titanium, or a stack oftitanium, aluminum, and titanium that are layered in that order.

The insulating layer 82 and the like may each be a multilayer. Forexample, as illustrated in FIG. 12B, the conductor 81 has a differencein level in the case where the insulating layer 82 includes insulatinglayers 82 a and 82 b that have different etching rates. In the casewhere another insulating layer used as an interlayer insulating film ora planarization film is a multilayer, the conductor 81 also has adifference in level. Although the insulating layer 82 is formed usingtwo layers here, the insulating layer 82 and another insulating layermay each be formed using three or more layers.

Note that the partition wall 67 can be formed using an inorganicinsulator, an insulating organic resin, or the like. The partition wall67 may be colored black or the like in order to shield the transistorsand the like from light and/or to determine the area of alight-receiving portion in each pixel.

Alternatively, a PIN diode element or the like formed using an amorphoussilicon film, a microcrystalline silicon film, or the like may be usedas the photoelectric conversion element 60.

FIG. 13 illustrates an example in which a thin film PIN photodiode isused as the photoelectric conversion element 60. In the photodiode, ap-type semiconductor layer 65, an i-type semiconductor layer 64, and ann-type semiconductor layer 63 are stacked in that order. The i-typesemiconductor layer 64 is preferably formed using amorphous silicon. Then-type semiconductor layer 63 and the p-type semiconductor layer 65 caneach be formed using amorphous silicon, microcrystalline silicon, or thelike that includes a dopant imparting the corresponding conductivitytype. A photodiode in which a photoelectric conversion layer is formedusing amorphous silicon has high sensitivity in a visible lightwavelength region, and therefore can easily sense weak visible light.

In the photoelectric conversion element 60 in FIG. 13, the p-typesemiconductor layer 65 is electrically connected to the electrode 66that is electrically connected to the transistors 51 and 53.Furthermore, the n-type semiconductor layer 63 is electrically connectedto the wiring 72 through the conductor 81.

Furthermore, any of examples illustrated in FIGS. 14A to 14F may beapplied to the structure of the photoelectric conversion element 60having a configuration of a PIN thin film photodiode and the connectionbetween the photoelectric conversion element 60 and the wirings. Notethat the structure of the photoelectric conversion element 60 and theconnection between the photoelectric conversion element 60 and thewirings are not limited thereto, and other configurations may beapplied.

FIG. 14A illustrates a structure provided with the light-transmittingconductive layer 62 in contact with the n-type semiconductor layer 63 ofthe photoelectric conversion element 60. The light-transmittingconductive layer 62 functions as an electrode and can increase theoutput current of the photoelectric conversion element 60.

For the light-transmitting conductive layer 62, the following can beused: indium tin oxide; indium tin oxide containing silicon; indiumoxide containing zinc; zinc oxide; zinc oxide containing gallium; zincoxide containing aluminum; tin oxide; tin oxide containing fluorine; tinoxide containing antimony; graphene; or the like. The light-transmittingconductive layer 62 is not limited to a single layer, and may be astacked layer of different films.

FIG. 14B illustrates a structure in which the n-type semiconductor layer63 of the photoelectric conversion element 60 is directly connected tothe wiring 88.

FIG. 14C illustrates a structure in which the light-transmittingconductive layer 62 in contact with the n-type semiconductor layer 63 ofthe photoelectric conversion element 60 is provided, and the wiring 88is electrically connected to the light-transmitting conductive layer 62.

FIG. 14D illustrates a structure in which an opening portion exposingthe n-type semiconductor layer 63 is provided in an insulating layercovering the photoelectric conversion element 60, and thelight-transmitting conductive layer 62 that covers the opening portionis electrically connected to the wiring 88.

FIG. 14E illustrates a structure provided with the conductor 81 thatpenetrates the photoelectric conversion element 60. In the structure,the wiring 72 is electrically connected to the n-type semiconductorlayer 63 through the conductor 81. Note that in the drawing, the wiring72 appears to be electrically connected to the electrode 66 through thep-type semiconductor layer 65. However, resistance in the lateraldirection of the p-type semiconductor layer 65 is high; therefore, whenan appropriate distance is provided between the wiring 72 and theelectrode 66, the resistance between the wiring 72 and the electrode 66is extremely high. Thus, the photoelectric conversion element 60 canhave diode characteristics without a short circuit between the anode andthe cathode. Note that two or more conductors 81 that are electricallyconnected to the n-type semiconductor layer 63 may be provided.

FIG. 14F illustrates a structure in which the photoelectric conversionelement 60 in FIG. 14E is provided with the light-transmittingconductive layer 62 in contact with the n-type semiconductor layer 63.

Note that each of the photoelectric conversion elements 60 in FIGS. 14Dto 14F has an advantage of having a large light-receiving area becausewirings and the like do not overlap with a light-receiving region.

Alternatively, as illustrated in FIG. 15, the photoelectric conversionelement 60 can be a photodiode including a silicon substrate 30 as aphotoelectric conversion layer.

The photoelectric conversion element 60 formed using the selenium-basedmaterial, amorphous silicon, or the like can be formed through generalsemiconductor manufacturing processes such as a deposition process, alithography process, and an etching process. Furthermore, the resistanceof the selenium-based material is high; thus, a structure in which thephotoelectric conversion layer 61 is not divided between the circuitscan be employed as illustrated in FIG. 9A. Therefore, the imaging devicein one embodiment of the present invention can be manufactured with ahigh yield at low cost. Meanwhile, to form a photodiode including thesilicon substrate 30 as the photoelectric conversion layer, processeswith high difficulty, such as a polishing process and a bonding process,are needed.

Furthermore, the imaging device in one embodiment of the presentinvention may be stacked over the silicon substrate 30 includingcircuits. For example, as illustrated in FIG. 16A, the pixel circuit mayoverlap with a layer 1400 that includes transistors 31 and 32 whoseactive regions are formed in the silicon substrate 30.

The circuit formed in the silicon substrate 30 is capable of reading asignal output from the pixel circuit and converting the signal; forexample, the circuit can include a CMOS inverter as illustrated in thecircuit diagram in FIG. 16B. A gate electrode of the transistor 31(n-channel transistor) is electrically connected to a gate electrode ofthe transistor 32 (p-channel transistor). One of a source electrode anda drain electrode of one transistor is electrically connected to one ofa source electrode and a drain electrode of the other transistor. Theother of the source electrode and the drain electrode of the onetransistor is electrically connected to a wiring, and the other of thesource electrode and the drain electrode of the other transistor iselectrically connected to another wiring.

Furthermore, the silicon substrate 30 is not limited to a bulk siliconsubstrate and can be a substrate made of germanium, silicon germanium,silicon carbide, gallium arsenide, aluminum gallium arsenide, indiumphosphide, gallium nitride, or an organic semiconductor.

As illustrated in FIG. 16C, the transistors 31 and 32 may each be atransistor including an active layer 35 formed using a silicon thinfilm. The active layer 35 can be formed using polycrystalline silicon orsingle crystal silicon of a silicon-on-insulator (SOI) structure.

Here, as illustrated in FIG. 15 and FIG. 16A, an insulating layer 80 isprovided between a region including an oxide semiconductor transistorand a region including a Si transistor (a Si photodiode in FIG. 15).

Dangling bonds of silicon are terminated with hydrogen in insulatinglayers provided in the vicinities of the active regions of thetransistors 31 and 32. Therefore, hydrogen has an effect of improvingthe reliability of the transistors 31 and 32. Meanwhile, hydrogen ininsulating layers which are provided in the vicinity of the oxidesemiconductor layer that is the active layer of the transistor 51 or thelike causes generation of carriers in the oxide semiconductor layer.Therefore, hydrogen might reduce the reliability of the transistor 51 orthe like. Consequently, in the case where one layer including thetransistor formed using a silicon-based semiconductor material and theother layer including the OS transistor are stacked, it is preferablethat the insulating layer 80 having a function of preventing diffusionof hydrogen be provided between the layers. Hydrogen is confined in theone layer by the insulating layer 80, so that the reliability of thetransistors 31 and 32 can be improved. Furthermore, diffusion ofhydrogen from the one layer to the other layer is inhibited, so that thereliability of the transistor 51 or the like can also be improved.

The insulating layer 80 can be, for example, formed using aluminumoxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttriumoxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ).

Note that as illustrated in FIG. 16A, a circuit (e.g., a driver circuit)formed using the silicon substrate 30, the transistor 51 or the like,and the photoelectric conversion element 60 can overlap with each other;thus, the integration degree of pixels can be increased. In other words,the resolution of the imaging device can be increased. For example, theimaging device is suitable for an imaging device whose number of pixelsis 4K2K, 8K4K, 16K8K, or the like. Note that the transistor 52, thetransistor 54, and the like included in the pixel circuit may have aregion that overlaps with the transistor 51 or the like and thephotoelectric conversion element 60.

In the imaging device in FIG. 16A, no photoelectric conversion elementis provided on the silicon substrate 30. Therefore, an optical path forthe photoelectric conversion element 60 can be secured without beinginfluenced by the transistors or wirings, and a pixel with a highaperture ratio can be formed.

The imaging device in one embodiment of the present invention can have astructure in FIG. 17.

The imaging device in FIG. 17 is a modification example of the imagingdevice in FIG. 16A. A CMOS inverter is formed using an OS transistor anda Si transistor.

Here, the transistor 32 is a p-channel Si transistor provided in thelayer 1400, and the transistor 31 is an n-channel OS transistor providedin the layer 1100. When only the p-channel transistor is provided on thesilicon substrate 30, a step of forming a well, an n-type impuritylayer, or the like can be skipped.

Although selenium is used for the photoelectric conversion element 60 inthe imaging device in FIG. 17, a PIN thin film photodiode may be used asin FIG. 13.

In the imaging device in FIG. 17, the transistor 31 can be formed in thesame process as the transistors 51 and 53 formed in the layer 1100.Thus, the manufacturing process of the imaging device can be simplified.

Note that the structure of the transistor and the photoelectricconversion element included in each of the imaging devices described inthis embodiment is only an example. Therefore, for example, one or moreof the transistors 51 to 54 can be formed using a transistor in which anactive region or an active layer includes silicon or the like.Furthermore, either one or both the transistors 31 and 32 can be formedusing a transistor including an oxide semiconductor layer as an activelayer.

FIG. 18A is a cross-sectional view of an example of a mode in which acolor filter and the like are added to the imaging device. Thecross-sectional view illustrates part of a region including pixelcircuits for three pixels. An insulating layer 2500 is formed over thelayer 1200 where the photoelectric conversion element 60 is formed. Asthe insulating layer 2500, for example, a silicon oxide film with a highvisible-light transmitting property can be used. In addition, a siliconnitride film may be stacked as a passivation film. Furthermore, adielectric film of hafnium oxide or the like may be stacked as ananti-reflection film.

A light-blocking layer 2510 may be formed over the insulating layer2500. The light-blocking layer 2510 has a function of inhibiting colormixing of light passing through the color filter. The light-blockinglayer 2510 can be formed using a metal layer of aluminum, tungsten, orthe like, or a stack including the metal layer and a dielectric filmfunctioning as an anti-reflection film.

An organic resin layer 2520 can be formed as a planarization film overthe insulating layer 2500 and the light-blocking layer 2510. A colorfilter 2530 is formed in each pixel. For example, a color filter 2530 a,a color filter 2530 b, and a color filter 2530 c each have a color ofred (R), green (G), blue (B), yellow (Y), cyan (C), magenta (M), or thelike, so that a color image can be obtained.

A light-transmitting insulating layer 2560 or the like can be providedover the color filter 2530.

As illustrated in FIG. 18B, an optical conversion layer 2550 may be usedinstead of the color filter 2530. Such a structure enables the imagingdevice to capture images in various wavelength regions.

For example, when a filter that blocks light having a wavelength shorterthan or equal to that of visible light is used as the optical conversionlayer 2550, an infrared imaging device can be obtained. When a filterthat blocks light having a wavelength shorter than or equal to that ofnear infrared light is used as the optical conversion layer 2550, a farinfrared imaging device can be obtained. When a filter that blocks lighthaving a wavelength longer than or equal to that of visible light isused as the optical conversion layer 2550, an ultraviolet imaging devicecan be obtained.

Furthermore, when a scintillator is used as the optical conversion layer2550, an imaging device that captures an image visualizing the intensityof radiation and is used for an X-ray imaging device, for example, canbe obtained. Radiation such as X-rays passes through an object to entera scintillator, and then is converted into light (fluorescence) such asvisible light or ultraviolet light owing to a phenomenon known asphotoluminescence. Then, the photoelectric conversion element 60 detectsthe light to obtain image data. Furthermore, the imaging device havingthe structure may be used in a radiation detector or the like.

A scintillator contains a substance that, when irradiated with radiationsuch as X-rays or gamma-rays, absorbs energy of the radiation to emitvisible light or ultraviolet light. For example, a resin or ceramics inwhich any of Gd₂O₂S:Tb, Gd₂O₂S:Pr, Gd₂O₂S:Eu, BaFCl:Eu, NaI, CsI, CaF₂,BaF₂, CeF₃, LiF, LiI, and ZnO is dispersed can be used.

In the photoelectric conversion element 60 including a selenium-basedmaterial, radiation such as X-rays can be directly converted intocharge; thus, the scintillator is not necessarily used.

A microlens array 2540 may be provided over the color filters 2530 a,2530 b, and 2530 c. Light penetrating lenses included in the microlensarray 2540 goes through the color filters positioned thereunder to reachthe photoelectric conversion element 60. Note that a region other thanthe layer 1200 in FIGS. 18A to 18C is referred to as a layer 1600.

The specific structure of the imaging device in FIG. 18C is illustratedin FIG. 19 by taking an example of the imaging device in FIG. 9A. Inaddition, the specific structure of the imaging device in FIG. 18C isillustrated in FIG. 20 by taking an example of the imaging device inFIG. 15.

The imaging device in one embodiment of the present invention may becombined with a diffraction grating 1500 as illustrated in FIG. 21 andFIG. 22. An image of an object through the diffraction grating 1500(i.e., a diffraction pattern) can be scanned into a pixel, and an inputimage (an object image) can be formed from a captured image in the pixelby arithmetic processing. In addition, the use of the diffractiongrating 1500 instead of a lens can reduce the cost of the imagingdevice.

The diffraction grating 1500 can be formed using a light-transmittingmaterial. An inorganic insulating film such as a silicon oxide film or asilicon oxynitride film can be used, for example. Alternatively, anorganic insulating film such as an acrylic resin film or a polyimideresin film may be used. Alternatively, a stack of the inorganicinsulating film and the organic insulating film may be used.

In addition, the diffraction grating 1500 can be formed by a lithographyprocess using a photosensitive resin or the like. Alternatively, thediffraction grating 1500 can be formed by a lithography process and anetching process. Alternatively, the diffraction grating 1500 can beformed by nanoimprint lithography, laser scribing, or the like.

Note that a space X may be provided between the diffraction grating 1500and the microlens array 2540. The space X can be less than or equal to 1mm, preferably less than or equal to 100 μm. The space may be an emptyspace or may be a sealing layer or an adhesion layer formed using alight-transmitting material. For example, an inert gas such as nitrogenor a rare gas can be sealed in the space. Alternatively, an acrylicresin, an epoxy resin, a polyimide resin, or the like may be provided inthe space. Alternatively, a liquid such as silicone oil may be provided.Even in the case where the microlens array 2540 is not provided, thespace X may be provided between the color filter 2530 and thediffraction grating 1500.

As illustrated in FIGS. 23A1 and 23B1, the imaging device may be bent.FIG. 23A1 illustrates a state in which the imaging device is bent in thedirection of dashed-two dotted line X1-X2. FIG. 23A2 is across-sectional view illustrating a portion indicated by dashed-twodotted line X1-X2 in FIG. 23A1. FIG. 23A3 is a cross-sectional viewillustrating a portion indicated by dashed-two dotted line Y1-Y2 in FIG.23A1.

FIG. 23B1 illustrates a state where the imaging device is bent in thedirection of dashed-two dotted line X3-X4 and the direction ofdashed-two dotted line Y3-Y4. FIG. 23B2 is a cross-sectional viewillustrating a portion indicated by dashed-two dotted line X3-X4 in FIG.23B1. FIG. 23B3 is a cross-sectional view illustrating a portionindicated by dashed-two dotted line Y3-Y4 in FIG. 23B1.

Bending the imaging device can reduce field curvature and astigmatism.Thus, the optical design of the lens and the like, which are used incombination of the imaging device, can be facilitated. For example, thenumber of lenses used for aberration correction can be reduced;accordingly, the size or weight of semiconductor devices including theimaging device can be easily reduced. In addition, the quality ofcaptured images can be improved.

In this embodiment, one embodiment of the present invention has beendescribed. Other embodiments of the present invention are described inthe other embodiments. Note that one embodiment of the present inventionis not limited thereto. In other words, various embodiments of theinvention are described in this embodiment and the other embodiments,and one embodiment of the present invention is not limited to aparticular embodiment. Although an example in which one embodiment ofthe present invention is applied to an imaging device is described, oneembodiment of the present invention is not limited thereto. Depending oncircumstances or conditions, one embodiment of the present invention isnot necessarily applied to an imaging device. One embodiment of thepresent invention may be applied to a semiconductor device with anotherfunction, for example. Although an example in which a channel formationregion, a source region, a drain region, or the like of a transistorincludes an oxide semiconductor is described as one embodiment of thepresent invention, one embodiment of the present invention is notlimited thereto. Depending on circumstances or conditions, varioustransistors or a channel formation region, a source region, a drainregion, or the like of a transistor in one embodiment of the presentinvention may include various semiconductors. Depending on circumstancesor conditions, various transistors or a channel formation region, asource region, a drain region, or the like of a transistor in oneembodiment of the present invention may include, for example, at leastone of silicon, germanium, silicon germanium, silicon carbide, galliumarsenide, aluminum gallium arsenide, indium phosphide, gallium nitride,and an organic semiconductor. Alternatively, for example, depending oncircumstances or conditions, various transistors or a channel formationregion, a source region, a drain region, or the like of a transistor inone embodiment of the present invention does not necessarily include anoxide semiconductor. For example, depending on circumstances orconditions, either one or both the transistors 51 and 53 do notnecessarily include an oxide semiconductor in the active layer.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 2

In this embodiment, a transistor including an oxide semiconductor thatcan be used in one embodiment of the present invention is described withreference to drawings. In the drawings in this embodiment, somecomponents are enlarged, reduced in size, or omitted for easyunderstanding.

FIGS. 24A and 24B are a top view and a cross-sectional view illustratinga transistor 101 in one embodiment of the present invention. FIG. 24A isa top view, and a cross section in the direction of dashed-dotted lineB1-B2 in FIG. 24A is illustrated in FIG. 24B. A cross section in thedirection of dashed-dotted line B3-B4 in FIG. 24A is illustrated in FIG.26A. The direction of dashed-dotted line B1-B2 is referred to as achannel length direction, and the direction of dashed-dotted line B3-B4is referred to as a channel width direction.

The transistor 101 includes an insulating layer 120 in contact with asubstrate 115; an oxide semiconductor layer 130 in contact with theinsulating layer 120; conductive layers 140 and 150 electricallyconnected to the oxide semiconductor layer 130; an insulating layer 160in contact with the oxide semiconductor layer 130 and the conductivelayers 140 and 150; a conductive layer 170 in contact with theinsulating layer 160; an insulating layer 175 in contact with theconductive layers 140 and 150, the insulating layer 160, and theconductive layer 170; and an insulating layer 180 in contact with theinsulating layer 175. The insulating layer 180 may function as aplanarization film as necessary.

The conductive layer 140, the conductive layer 150, the insulating layer160, and the conductive layer 170 can function as a source electrodelayer, a drain electrode layer, a gate insulating film, and a gateelectrode layer, respectively.

A region 231, a region 232, and a region 233 in FIG. 24B can function asa source region, a drain region, and a channel formation region,respectively. The region 231 and the region 232 are in contact with theconductive layer 140 and the conductive layer 150, respectively. When aconductive material that is easily bonded to oxygen is used for theconductive layers 140 and 150, the resistance of the regions 231 and 232can be reduced.

Specifically, since the oxide semiconductor layer 130 is in contact withthe conductive layers 140 and 150, an oxygen vacancy is generated in theoxide semiconductor layer 130, and interaction between the oxygenvacancy and hydrogen that remains in the oxide semiconductor layer 130or diffuses into the oxide semiconductor layer 130 from the outsidechanges the regions 231 and 232 to n-type regions with low resistance.

Note that functions of a “source” and a “drain” of a transistor aresometimes interchanged with each other when a transistor of an oppositeconductivity type is used or when the direction of current flow ischanged in circuit operation, for example. Therefore, the terms “source”and “drain” can be interchanged with each other in this specification.In addition, the term “electrode layer” can be changed into the term“wiring.”

The conductive layer 170 includes two layers, conductive layers 171 and172, but also may be a single layer or a stack of three or more layers.The same applies to other transistors described in this embodiment.

Each of the conductive layers 140 and 150 is a single layer, but alsomay be a stack of two or more layers. The same applies to othertransistors described in this embodiment.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 24C and 24D. FIG. 24C is a top view of atransistor 102. A cross section in the direction of dashed-dotted lineC1-C2 in FIG. 24C is illustrated in FIG. 24D. A cross section in thedirection of dashed-dotted line C3-C4 in FIG. 24C is illustrated in FIG.26B. The direction of dashed-dotted line C1-C2 is referred to as achannel length direction, and the direction of dashed-dotted line C3-C4is referred to as a channel width direction.

The transistor 102 has the same structure as the transistor 101 exceptthat an end portion of the insulating layer 160 functioning as a gateinsulating film is not aligned with an end portion of the conductivelayer 170 functioning as a gate electrode layer. In the transistor 102,wide areas of the conductive layers 140 and 150 are covered with theinsulating layer 160 and accordingly the resistance between theconductive layer 170 and the conductive layers 140 and 150 is high;therefore, the transistor 102 has low gate leakage current.

The transistors 101 and 102 each have a top-gate structure including aregion where the conductive layer 170 overlaps with the conductivelayers 140 and 150. To reduce parasitic capacitance, the width of theregion in the channel length direction is preferably greater than orequal to 3 nm and less than 300 nm. Since an offset region is not formedin the oxide semiconductor layer 130 in this structure, a transistorwith high on-state current can be easily formed.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 24E and 24F. FIG. 24E is a top view of atransistor 103. A cross section in the direction of dashed-dotted lineD1-D2 in FIG. 24E is illustrated in FIG. 24F. A cross section in thedirection of dashed-dotted line D3-D4 in FIG. 24E is illustrated in FIG.26A. The direction of dashed-dotted line D1-D2 is referred to as achannel length direction, and the direction of dashed-dotted line D3-D4is referred to as a channel width direction.

The transistor 103 includes the insulating layer 120 in contact with thesubstrate 115; the oxide semiconductor layer 130 in contact with theinsulating layer 120; the insulating layer 160 in contact with the oxidesemiconductor layer 130; the conductive layer 170 in contact with theinsulating layer 160; the insulating layer 175 covering the oxidesemiconductor layer 130, the insulating layer 160, and the conductivelayer 170; the insulating layer 180 in contact with the insulating layer175; and the conductive layers 140 and 150 electrically connected to theoxide semiconductor layer 130 through openings provided in theinsulating layers 175 and 180. The transistor 103 may further include,for example, an insulating layer (planarization film) in contact withthe insulating layer 180 and the conductive layers 140 and 150 asnecessary.

The conductive layer 140, the conductive layer 150, the insulating layer160, and the conductive layer 170 can function as a source electrodelayer, a drain electrode layer, a gate insulating film, and a gateelectrode layer, respectively.

The region 231, the region 232, and the region 233 in FIG. 24F canfunction as a source region, a drain region, and a channel formationregion, respectively. The regions 231 and 232 are in contact with theinsulating layer 175. When an insulating material containing hydrogen isused for the insulating layer 175, for example, the resistance of theregions 231 and 232 can be reduced.

Specifically, interaction between an oxygen vacancy generated in theregions 231 and 232 by the steps up to formation of the insulating layer175 and hydrogen that diffuses into the regions 231 and 232 from theinsulating layer 175 changes the regions 231 and 232 to n-type regionswith low resistance. As the insulating material containing hydrogen, forexample, silicon nitride, aluminum nitride, or the like can be used.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 25A and 25B. FIG. 25A is a top view of atransistor 104. A cross section in the direction of dashed-dotted lineE1-E2 in FIG. 25A is illustrated in FIG. 25B. A cross section in thedirection of dashed-dotted line E3-E4 in FIG. 25A is illustrated in FIG.26A. The direction of dashed-dotted line E1-E2 is referred to as achannel length direction, and the direction of dashed-dotted line E3-E4is referred to as a channel width direction.

The transistor 104 has the same structure as the transistor 103 exceptthat the conductive layers 140 and 150 in contact with the oxidesemiconductor layer 130 cover end portions of the oxide semiconductorlayer 130.

In FIG. 25B, regions 331 and 334 can function as a source region,regions 332 and 335 can function as a drain region, and a region 333 canfunction as a channel formation region.

The resistance of the regions 331 and 332 can be reduced in a mannersimilar to that of the regions 231 and 232 in the transistor 101.

The resistance of the regions 334 and 335 can be reduced in a mannersimilar to that of the regions 231 and 232 in the transistor 103. In thecase where the length of the regions 334 and 335 in the channel lengthdirection is less than or equal to 100 nm, preferably less than or equalto 50 nm, a gate electric field prevents a significant decrease inon-state current. Therefore, a reduction in resistance of the regions334 and 335 is not performed in some cases.

The transistors 103 and 104 each have a self-aligned structure that doesnot include a region where the conductive layer 170 overlaps with theconductive layers 140 and 150. A transistor with a self-alignedstructure, which has extremely low parasitic capacitance between a gateelectrode layer and source and drain electrode layers, is suitable forapplications that require high-speed operation.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 25C and 25D. FIG. 25C is a top view of atransistor 105. A cross section in the direction of dashed-dotted lineF1-F2 in FIG. 25C is illustrated in FIG. 25D. A cross section in thedirection of dashed-dotted line F3-F4 in FIG. 25C is illustrated in FIG.26A. The direction of dashed-dotted line F1-F2 is referred to as achannel length direction, and the direction of dashed-dotted line F3-F4is referred to as a channel width direction.

The transistor 105 includes the insulating layer 120 in contact with thesubstrate 115; the oxide semiconductor layer 130 in contact with theinsulating layer 120; conductive layers 141 and 151 electricallyconnected to the oxide semiconductor layer 130; the insulating layer 160in contact with the oxide semiconductor layer 130 and the conductivelayers 141 and 151; the conductive layer 170 in contact with theinsulating layer 160; the insulating layer 175 in contact with the oxidesemiconductor layer 130, the conductive layers 141 and 151, theinsulating layer 160, and the conductive layer 170; the insulating layer180 in contact with the insulating layer 175; and conductive layers 142and 152 electrically connected to the conductive layers 141 and 151,respectively, through openings provided in the insulating layers 175 and180. The transistor 105 may further include, for example, an insulatinglayer in contact with the insulating layer 180 and the conductive layers142 and 152 as necessary.

Here, the conductive layers 141 and 151 are in contact with the topsurface of the oxide semiconductor layer 130 and are not in contact withside surfaces of the oxide semiconductor layer 130.

The transistor 105 has the same structure as the transistor 101 exceptthat the conductive layers 141 and 151 are provided, that openings areprovided in the insulating layers 175 and 180, and that the conductivelayers 142 and 152 electrically connected to the conductive layers 141and 151, respectively, through the openings are provided. The conductivelayer 140 (the conductive layers 141 and 142) can function as a sourceelectrode layer, and the conductive layer 150 (the conductive layers 151and 152) can function as a drain electrode layer.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 25E and 25F. FIG. 25E is a top view of atransistor 106. A cross section in the direction of dashed-dotted lineG1-G2 in FIG. 25E is illustrated in FIG. 25F. A cross section in thedirection of dashed-dotted line G3-G4 in FIG. 25E is illustrated in FIG.26A. The direction of dashed-dotted line G1-G2 is referred to as achannel length direction, and the direction of dashed-dotted line G3-G4is referred to as a channel width direction.

The transistor 106 includes the insulating layer 120 in contact with thesubstrate 115; the oxide semiconductor layer 130 in contact with theinsulating layer 120; the conductive layers 141 and 151 electricallyconnected to the oxide semiconductor layer 130; the insulating layer 160in contact with the oxide semiconductor layer 130; the conductive layer170 in contact with the insulating layer 160; the insulating layer 175in contact with the insulating layer 120, the oxide semiconductor layer130, the conductive layers 141 and 151, the insulating layer 160, andthe conductive layer 170; the insulating layer 180 in contact with theinsulating layer 175; and the conductive layers 142 and 152 electricallyconnected to the conductive layers 141 and 151, respectively, throughopenings provided in the insulating layers 175 and 180. The transistor106 may further include, for example, an insulating layer (planarizationfilm) in contact with the insulating layer 180 and the conductive layers142 and 152 as necessary.

The conductive layers 141 and 151 are in contact with the top surface ofthe oxide semiconductor layer 130 and are not in contact with sidesurfaces of the oxide semiconductor layer 130.

The transistor 106 has the same structure as the transistor 103 exceptthat the conductive layers 141 and 151 are provided. The conductivelayer 140 (the conductive layers 141 and 142) can function as a sourceelectrode layer, and the conductive layer 150 (the conductive layers 151and 152) can function as a drain electrode layer.

In the structures of the transistors 105 and 106, the conductive layers140 and 150 are not in contact with the insulating layer 120. Thesestructures make the insulating layer 120 less likely to be deprived ofoxygen by the conductive layers 140 and 150 and facilitate oxygen supplyfrom the insulating layer 120 to the oxide semiconductor layer 130.

An impurity for forming an oxygen vacancy to increase conductivity maybe added to the regions 231 and 232 in the transistor 103 and theregions 334 and 335 in the transistors 104 and 106. As an impurity forforming an oxygen vacancy in an oxide semiconductor layer, for example,one or more of the following can be used: phosphorus, arsenic, antimony,boron, aluminum, silicon, nitrogen, helium, neon, argon, krypton, xenon,indium, fluorine, chlorine, titanium, zinc, and carbon. As a method foradding the impurity, plasma treatment, ion implantation, ion doping,plasma immersion ion implantation, or the like can be used.

When the above element is added as an impurity element to the oxidesemiconductor layer, a bond between a metal element and oxygen in theoxide semiconductor layer is cut, so that an oxygen vacancy is formed.Interaction between an oxygen vacancy in the oxide semiconductor layerand hydrogen that remains in the oxide semiconductor layer or is addedto the oxide semiconductor layer later can increase the conductivity ofthe oxide semiconductor layer.

When hydrogen is added to an oxide semiconductor in which an oxygenvacancy is formed by addition of an impurity element, hydrogen enters anoxygen vacant site and forms a donor level in the vicinity of theconduction band. Consequently, an oxide conductor can be formed. Here,an oxide conductor refers to an oxide semiconductor having become aconductor. Note that the oxide conductor has a light-transmittingproperty in a manner similar to the oxide semiconductor.

The oxide conductor is a degenerated semiconductor and it is suggestedthat the conduction band edge equals or substantially equals the Fermilevel. For that reason, an ohmic contact is made between an oxideconductor layer and conductive layers functioning as a source electrodelayer and a drain electrode layer; thus, contact resistance between theoxide conductor layer and the conductive layers functioning as a sourceelectrode layer and a drain electrode layer can be reduced.

The transistor in one embodiment of the present invention may include aconductive layer 173 between the oxide semiconductor layer 130 and thesubstrate 115 as illustrated in cross-sectional views in the channellength direction in FIGS. 27A to 27F and cross-sectional views in thechannel width direction in FIGS. 26C and 26D. When the conductive layer173 is used as a second gate electrode layer (back gate), the on-statecurrent can be increased or the threshold voltage can be controlled. Inthe cross-sectional views in FIGS. 27A to 27F, the width of theconductive layer 173 may be shorter than that of the oxide semiconductorlayer 130. Moreover, the width of the conductive layer 173 may beshorter than that of the conductive layer 170.

In order to increase the on-state current, for example, the conductivelayers 170 and 173 are made to have the same potential, and thetransistor is driven as a double-gate transistor. Furthermore, in orderto control the threshold voltage, a fixed potential that is differentfrom the potential of the conductive layer 170 is applied to theconductive layer 173. To set the conductive layers 170 and 173 at thesame potential, for example, as illustrated in FIG. 26D, the conductivelayers 170 and 173 may be electrically connected to each other through acontact hole.

Although the transistors 101 to 106 in FIGS. 24A to 24F and FIGS. 25A to25F are examples in which the oxide semiconductor layer 130 is a singlelayer, the oxide semiconductor layer 130 may be a stacked layer. Theoxide semiconductor layer 130 in the transistors 101 to 106 can bereplaced with the oxide semiconductor layer 130 in FIG. 28B, 28C, 28D,or 28E.

FIG. 28A is a top view of the oxide semiconductor layer 130, and FIGS.28B and 28C are cross-sectional views of the oxide semiconductor layer130 with a two-layer structure. FIGS. 28D and 28E are cross-sectionalviews of the oxide semiconductor layer 130 with a three-layer structure.

Oxide semiconductor layers with different compositions, for example, canbe used as an oxide semiconductor layer 130 a, an oxide semiconductorlayer 130 b, and an oxide semiconductor layer 130 c.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 29A and 29B. FIG. 29A is a top view of atransistor 107. A cross section in the direction of dashed-dotted lineH1-H2 in FIG. 29A is illustrated in FIG. 29B. A cross section in thedirection of dashed-dotted line H3-H4 in FIG. 29A is illustrated in FIG.31A. The direction of dashed-dotted line H1-H2 is referred to as achannel length direction, and the direction of dashed-dotted line H3-H4is referred to as a channel width direction.

The transistor 107 includes the insulating layer 120 in contact with thesubstrate 115; a stack of the oxide semiconductor layers 130 a and 130 bin contact with the insulating layer 120; the conductive layers 140 and150 electrically connected to the stack; the oxide semiconductor layer130 c in contact with the stack and the conductive layers 140 and 150;the insulating layer 160 in contact with the oxide semiconductor layer130 c; the conductive layer 170 in contact with the insulating layer160; the insulating layer 175 in contact with the conductive layers 140and 150, the oxide semiconductor layer 130 c, the insulating layer 160,and the conductive layer 170; and the insulating layer 180 in contactwith the insulating layer 175. The insulating layer 180 may function asa planarization film as necessary.

The transistor 107 has the same structure as the transistor 101 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 231 and 232, thatthe oxide semiconductor layer 130 includes three layers (the oxidesemiconductor layers 130 a to 130 c) in the region 233, and that part ofthe oxide semiconductor layer (the oxide semiconductor layer 130 c)exists between the insulating layer 160 and the conductive layers 140and 150.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 29C and 29D. FIG. 29C is a top view of atransistor 108. A cross section in the direction of dashed-dotted line11-12 in FIG. 29C is illustrated in FIG. 29D. A cross section in thedirection of dashed-dotted line 13-14 in FIG. 29C is illustrated in FIG.31B. The direction of dashed-dotted line 11-12 is referred to as achannel length direction, and the direction of dashed-dotted line 13-14is referred to as a channel width direction.

The transistor 108 differs from the transistor 107 in that end portionsof the insulating layer 160 and the oxide semiconductor layer 130 c arenot aligned with the end portion of the conductive layer 170.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 29E and 29F. FIG. 29E is a top view of atransistor 109. A cross section in the direction of dashed-dotted lineJ1-J2 in FIG. 29E is illustrated in FIG. 29F. A cross section in thedirection of dashed-dotted line J3-J4 in FIG. 29E is illustrated in FIG.31A. The direction of dashed-dotted line J1-J2 is referred to as achannel length direction, and the direction of dashed-dotted line J3-J4is referred to as a channel width direction.

The transistor 109 includes the insulating layer 120 in contact with thesubstrate 115; a stack of the oxide semiconductor layers 130 a and 130 bin contact with the insulating layer 120; the oxide semiconductor layer130 c in contact with the stack; the insulating layer 160 in contactwith the oxide semiconductor layer 130 c; the conductive layer 170 incontact with the insulating layer 160; the insulating layer 175 coveringthe stack, the oxide semiconductor layer 130 c, the insulating layer160, and the conductive layer 170; the insulating layer 180 in contactwith the insulating layer 175; and the conductive layers 140 and 150electrically connected to the stack through openings provided in theinsulating layers 175 and 180. The transistor 109 may further include,for example, an insulating layer (planarization film) in contact withthe insulating layer 180 and the conductive layers 140 and 150 asnecessary.

The transistor 109 has the same structure as the transistor 103 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 231 and 232 andthat the oxide semiconductor layer 130 includes three layers (the oxidesemiconductor layers 130 a to 130 c) in the region 233.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 30A and 30B. FIG. 30A is a top view of atransistor 110. A cross section in the direction of dashed-dotted lineK1-K2 in FIG. 30A is illustrated in FIG. 30B. A cross section in thedirection of dashed-dotted line K3-K4 in FIG. 30A is illustrated in FIG.31A. The direction of dashed-dotted line K1-K2 is referred to as achannel length direction, and the direction of dashed-dotted line K3-K4is referred to as a channel width direction.

The transistor 110 has the same structure as the transistor 104 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 331 and 332 andthat the oxide semiconductor layer 130 includes three layers (the oxidesemiconductor layers 130 a to 130 c) in the region 333.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 30C and 30D. FIG. 30C is a top view of atransistor 111. A cross section in the direction of dashed-dotted lineL1-L2 in FIG. 30C is illustrated in FIG. 30D. A cross section in thedirection of dashed-dotted line L3-L4 in FIG. 30C is illustrated in FIG.31A. The direction of dashed-dotted line L1-L2 is referred to as achannel length direction, and the direction of dashed-dotted line L3-L4is referred to as a channel width direction.

The transistor 111 includes the insulating layer 120 in contact with thesubstrate 115; a stack of the oxide semiconductor layers 130 a and 130 bin contact with the insulating layer 120; the conductive layers 141 and151 electrically connected to the stack; the oxide semiconductor layer130 c in contact with the stack and the conductive layers 141 and 151;the insulating layer 160 in contact with the oxide semiconductor layer130 c; the conductive layer 170 in contact with the insulating layer160; the insulating layer 175 in contact with the stack, the conductivelayers 141 and 151, the oxide semiconductor layer 130 c, the insulatinglayer 160, and the conductive layer 170; the insulating layer 180 incontact with the insulating layer 175; and the conductive layers 142 and152 electrically connected to the conductive layers 141 and 151,respectively, through openings provided in the insulating layers 175 and180. The transistor 111 may further include, for example, an insulatinglayer (planarization film) in contact with the insulating layer 180 andthe conductive layers 142 and 152 as necessary.

The transistor 111 has the same structure as the transistor 105 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 231 and 232, thatthe oxide semiconductor layer 130 includes three layers (the oxidesemiconductor layers 130 a to 130 c) in the region 233, and that part ofthe oxide semiconductor layer (the oxide semiconductor layer 130 c)exists between the insulating layer 160 and the conductive layers 141and 151.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 30E and 30F. FIG. 30E is a top view of atransistor 112. A cross section in the direction of dashed-dotted lineM1-M2 in FIG. 30E is illustrated in FIG. 30F. A cross section in thedirection of dashed-dotted line M3-M4 in FIG. 30E is illustrated in FIG.31A. The direction of dashed-dotted line M1-M2 is referred to as achannel length direction, and the direction of dashed-dotted line M3-M4is referred to as a channel width direction.

The transistor 112 has the same structure as the transistor 106 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 331, 332, 334, and335 and that the oxide semiconductor layer 130 includes three layers(the oxide semiconductor layers 130 a to 130 c) in the region 333.

The transistor in one embodiment of the present invention may includethe conductive layer 173 between the oxide semiconductor layer 130 andthe substrate 115 as illustrated in cross-sectional views in the channellength direction in FIGS. 32A to 32F and cross-sectional views in thechannel width direction in FIGS. 31C and 31D. When the conductive layeris used as a second gate electrode layer (back gate), the on-statecurrent can be increased or the threshold voltage can be controlled. Inthe cross-sectional views in FIGS. 32A to 32F, the width of theconductive layer 173 may be shorter than that of the oxide semiconductorlayer 130. Moreover, the width of the conductive layer 173 may beshorter than that of the conductive layer 170.

As illustrated in FIGS. 33A and 33B (illustrating only the oxidesemiconductor layer 130, the conductive layer 140, and the conductivelayer 150), the width (W_(SD)) of the conductive layer 140 (sourceelectrode layer) and the conductive layer 150 (drain electrode layer) inthe transistor of one embodiment of the present invention may be eitherlonger than or shorter than the width (W_(OS)) of the oxidesemiconductor layer 130. When W_(OS)≥W_(SD) (W_(SD) is less than orequal to W_(OS)) is satisfied, a gate electric field is easily appliedto the entire oxide semiconductor layer 130, so that electricalcharacteristics of the transistor can be improved. As illustrated inFIG. 33C, the conductive layers 140 and 150 may be formed only in aregion that overlaps with the oxide semiconductor layer 130.

In the transistor in one embodiment of the present invention (any of thetransistors 101 to 112), the conductive layer 170 functioning as a gateelectrode layer electrically surrounds the oxide semiconductor layer 130in the channel width direction with the insulating layer 160 functioningas a gate insulating film positioned therebetween. This structureincreases the on-state current. Such a transistor structure is referredto as a surrounded channel (s-channel) structure.

In the transistor including the oxide semiconductor layers 130 a and 130b and the transistor including the oxide semiconductor layers 130 a to130 c, selecting appropriate materials for the two or three layersforming the oxide semiconductor layer 130 makes current flow to theoxide semiconductor layer 130 b. Since current flows to the oxidesemiconductor layer 130 b, the current is hardly influenced by interfacescattering, leading to high on-state current. Therefore, increasing thethickness of the oxide semiconductor layer 130 b might increase theon-state current.

A semiconductor device including a transistor with any of the abovestructures can have favorable electrical characteristics.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 3

In this embodiment, components of the transistors described inEmbodiment 2 are described in detail.

As the substrate 115, a glass substrate, a quartz substrate, asemiconductor substrate, a ceramic substrate, a metal substrate having asurface subjected to insulation treatment, or the like can be used. Thesubstrate 115 can be a silicon substrate provided with a transistorand/or a photodiode; and an insulating layer, a wiring, a conductorfunctioning as a contact plug, and the like that are provided over thesilicon substrate. Note that when p-channel transistors are formed usingthe silicon substrate, a silicon substrate with n⁻-type conductivity ispreferably used. Alternatively, an SOI substrate including an n⁻-type ori-type silicon layer may be used. In the case where a p-channeltransistor is formed using the silicon substrate, a surface of thesilicon substrate where the transistor is formed preferably has a (110)plane orientation. Forming a p-channel transistor with the (110) planecan increase mobility.

The insulating layer 120 can have a function of supplying oxygen to theoxide semiconductor layer 130 as well as a function of preventingdiffusion of impurities from a component included in the substrate 115.For this reason, the insulating layer 120 is preferably an insulatingfilm containing oxygen and more preferably, the insulating layer 120 isan insulating film containing oxygen in which the oxygen content ishigher than that in the stoichiometric composition. The insulating layer120 is a film in which the amount of released oxygen when converted intooxygen atoms is preferably greater than or equal to 1.0×10¹⁹ atoms/cm³in TDS analysis. In the TDS analysis, the film surface temperature ishigher than or equal to 100° C. and lower than or equal to 700° C., orhigher than or equal to 100° C. and lower than or equal to 500° C. Inthe case where the substrate 115 is provided with another device, theinsulating layer 120 also functions as an interlayer insulating film. Inthat case, the insulating layer 120 is preferably subjected toplanarization treatment such as CMP so as to have a flat surface.

For example, the insulating layer 120 can be formed using an oxideinsulating film including aluminum oxide, magnesium oxide, siliconoxide, silicon oxynitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,tantalum oxide, or the like; a nitride insulating film including siliconnitride, silicon nitride oxide, aluminum nitride, aluminum nitrideoxide, or the like; or a mixed material of any of these. The insulatinglayer 120 may be a stack of any of the above materials.

In this embodiment, detailed description is given mainly on the casewhere the oxide semiconductor layer 130 of the transistor has athree-layer structure in which the oxide semiconductor layers 130 a to130 c are sequentially stacked from the insulating layer 120 side.

Note that in the case where the oxide semiconductor layer 130 is asingle layer, a layer corresponding to the oxide semiconductor layer 130b described in this embodiment is used.

In the case where the oxide semiconductor layer 130 has a two-layerstructure, a stack in which a layer corresponding to the oxidesemiconductor layer 130 a and a layer corresponding to the oxidesemiconductor layer 130 b are sequentially stacked from the insulatinglayer 120 side described in this embodiment is used. In such a case, theoxide semiconductor layers 130 a and 130 b can be replaced with eachother.

In the case where the oxide semiconductor layer 130 has a layeredstructure of four or more layers, for example, a structure in whichanother oxide semiconductor layer is added to the three-layer stack ofthe oxide semiconductor layer 130 described in this embodiment can beemployed.

For the oxide semiconductor layer 130 b, for example, an oxidesemiconductor whose electron affinity (an energy difference between avacuum level and the conduction band minimum) is higher than those ofthe oxide semiconductor layers 130 a and 130 c is used. The electronaffinity can be obtained by subtracting an energy difference between theconduction band minimum and the valence band maximum (what is called anenergy gap) from an energy difference between the vacuum level and thevalence band maximum (what is called an ionization potential).

The oxide semiconductor layers 130 a and 130 c each contain one or morekinds of metal elements contained in the oxide semiconductor layer 130b. For example, the oxide semiconductor layers 130 a and 130 c arepreferably formed using an oxide semiconductor whose conduction bandminimum is closer to a vacuum level than that of the oxide semiconductorlayer 130 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV orless.

In such a structure, when an electric field is applied to the conductivelayer 170, a channel is formed in the oxide semiconductor layer 130 bwhose conduction band minimum is the lowest in the oxide semiconductorlayer 130.

Furthermore, since the oxide semiconductor layer 130 a contains one ormore kinds of metal elements contained in the oxide semiconductor layer130 b, an interface state is unlikely to be formed at the interfacebetween the oxide semiconductor layers 130 a and 130 b, compared withthe interface between the oxide semiconductor layer 130 b and theinsulating layer 120 on the assumption that the oxide semiconductorlayer 130 b is in contact with the insulating layer 120. The interfacestate sometimes forms a channel; therefore, the threshold voltage of thetransistor is changed in some cases. Thus, with the oxide semiconductorlayer 130 a, variations in electrical characteristics of the transistor,such as a threshold voltage, can be reduced. Moreover, the reliabilityof the transistor can be improved.

Furthermore, since the oxide semiconductor layer 130 c contains one ormore kinds of metal elements contained in the oxide semiconductor layer130 b, scattering of carriers is unlikely to occur at the interfacebetween the oxide semiconductor layers 130 b and 130 c, compared withthe interface between the oxide semiconductor layer 130 b and the gateinsulating film (the insulating layer 160) on the assumption that theoxide semiconductor layer 130 b is in contact with the gate insulatingfilm. Thus, with the oxide semiconductor layer 130 c, the field-effectmobility of the transistor can be increased.

For the oxide semiconductor layers 130 a and 130 c, for example, amaterial containing Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf with ahigher atomic ratio than that used for the oxide semiconductor layer 130b can be used. Specifically, the atomic ratio of any of the above metalelements in the oxide semiconductor layers 130 a and 130 c is 1.5 timesor more, preferably 2 times or more, more preferably 3 times or more aslarge as that in the oxide semiconductor layer 130 b. Any of the abovemetal elements is strongly bonded to oxygen and thus has a function ofsuppressing generation of an oxygen vacancy in the oxide semiconductorlayers 130 a and 130 c. That is, an oxygen vacancy is less likely to begenerated in the oxide semiconductor layers 130 a and 130 c than in theoxide semiconductor layer 130 b.

An oxide semiconductor that can be used for each of the oxidesemiconductor layers 130 a to 130 c preferably contains at least In orZn. Both In and Zn are preferably contained. In order to reducevariations in electrical characteristics of the transistor including theoxide semiconductor, the oxide semiconductor preferably contains astabilizer in addition to In and Zn.

Examples of a stabilizer include Ga, Sn, Hf, Al, and Zr. Other examplesof the stabilizer include lanthanoids such as La, Ce, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

As the oxide semiconductor, for example, any of the following can beused: indium oxide, tin oxide, gallium oxide, zinc oxide, an In—Znoxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, anIn—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, anIn—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide,an In—Hf—Zn oxide, an In-La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Znoxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, anIn—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide,an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Znoxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide,an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide.

For example, an In—Ga—Zn oxide means an oxide containing In, Ga, and Znas its main components. The In—Ga—Zn oxide may contain another metalelement in addition to In, Ga, and Zn. In this specification, a filmcontaining the In—Ga—Zn oxide is also referred to as an IGZO film.

A material represented by InMO₃(ZnO)_(m) (m>0, where m is not aninteger) may be used. Note that M represents one or more metal elementsselected from Ga, Y, Zr, La, Ce, and Nd. Alternatively, a materialrepresented by In₂SnO₅(ZnO)_(n) (n>0, where n is an integer) may beused.

Note that when each of the oxide semiconductor layers 130 a to 130 c isan In-M-Zn oxide containing at least indium, zinc, and M (M is a metalsuch as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), in the case where theoxide semiconductor layer 130 a has an atomic ratio of In to M and Znwhich is x₁:y₁:z₁, the oxide semiconductor layer 130 b has an atomicratio of In to M and Zn which is x₂:y₂:z₂, and the oxide semiconductorlayer 130 c has an atomic ratio of In to M and Zn which is x₃:y₃:z₃,each of y₁/x₁ and y₃/x₃ is preferably larger than y₂/x₂. Each of y₁/x₁and y₃/x₃ is 1.5 times or more, preferably 2 times or more, morepreferably 3 times or more as large as y₂/x₂. At this time, when y₂ isgreater than or equal to x₂ in the oxide semiconductor layer 130 b, thetransistor can have stable electrical characteristics. However, when y₂is 3 times or more as large as x₂, the field-effect mobility of thetransistor is reduced; accordingly, y₂ is preferably smaller than 3times x₂.

In the case where Zn and O are not taken into consideration, theproportion of In and the proportion of M in each of the oxidesemiconductor layers 130 a and 130 c are preferably less than 50 atomic% and greater than or equal to 50 atomic %, respectively, morepreferably less than 25 atomic % and greater than or equal to 75 atomic%, respectively. Furthermore, in the case where Zn and O are not takeninto consideration, the proportion of In and the proportion of M in theoxide semiconductor layer 130 b are preferably greater than or equal to25 atomic % and less than 75 atomic %, respectively, more preferablygreater than or equal to 34 atomic % and less than 66 atomic %,respectively.

The indium content in the oxide semiconductor layer 130 b is preferablyhigher than those in the oxide semiconductor layers 130 a and 130 c. Inan oxide semiconductor, the s orbital of heavy metal mainly contributesto carrier transfer, and when the proportion of In in the oxidesemiconductor is increased, overlap of the s orbitals is likely to beincreased. Therefore, an oxide in which the proportion of In is higherthan that of M has higher mobility than an oxide in which the proportionof In is equal to or lower than that of M. Thus, with the use of anoxide having a high content of indium for the oxide semiconductor layer130 b, a transistor having high field-effect mobility can be obtained.

The thickness of the oxide semiconductor layer 130 a is greater than orequal to 3 nm and less than or equal to 100 nm, preferably greater thanor equal to 5 nm and less than or equal to 50 nm, more preferablygreater than or equal to 5 nm and less than or equal to 25 nm. Thethickness of the oxide semiconductor layer 130 b is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 5 nm and less than or equal to 150 nm, more preferablygreater than or equal to 10 nm and less than or equal to 100 nm. Thethickness of the oxide semiconductor layer 130 c is greater than orequal to 1 nm and less than or equal to 50 nm, preferably greater thanor equal to 2 nm and less than or equal to 30 nm, more preferablygreater than or equal to 3 nm and less than or equal to 15 nm. Inaddition, the oxide semiconductor layer 130 b is preferably thicker thanthe oxide semiconductor layer 130 c.

Note that in order that a transistor in which a channel is formed in anoxide semiconductor layer have stable electrical characteristics, it iseffective to make the oxide semiconductor layer intrinsic orsubstantially intrinsic by reducing the concentration of impurities inthe oxide semiconductor layer. The term “substantially intrinsic” refersto a state where an oxide semiconductor layer has a carrier densitylower than 1×10¹⁵/cm³, preferably lower than 1×10¹³/cm³, more preferablylower than 8×10¹¹/cm³, still more preferably higher than or equal to1×10⁻⁹/cm³ and lower than 1×10⁸/cm³.

In the oxide semiconductor layer, hydrogen, nitrogen, carbon, silicon,and a metal element other than main components of the oxidesemiconductor layer are impurities. For example, hydrogen and nitrogenform donor levels to increase the carrier density, and silicon formsimpurity levels in the oxide semiconductor layer. The impurity levelsserve as traps and might cause deterioration of electricalcharacteristics of the transistor. Therefore, it is preferable to reducethe concentration of the impurities in the oxide semiconductor layers130 a to 130 c and at interfaces between the oxide semiconductor layers.

In order to make the oxide semiconductor layer intrinsic orsubstantially intrinsic, the oxide semiconductor layer is controlled tohave a region in which the concentration of hydrogen estimated bysecondary ion mass spectrometry (SIMS) is lower than or equal to 2×10²⁰atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, morepreferably lower than or equal to 1×10¹⁹ atoms/cm³, still morepreferably higher than or equal to 1×10¹⁷ atoms/cm³ and lower than orequal to 5×10¹⁸ atoms/cm³. Furthermore, the oxide semiconductor layer iscontrolled to have a region in which the concentration of nitrogen islower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, more preferably lower than or equal to 1×10¹⁸ atoms/cm³,still more preferably higher than or equal to 5×10¹⁶ atoms/cm³ and lowerthan or equal to 5×10¹⁷ atoms/cm³.

The high concentration of silicon or carbon might reduce thecrystallinity of the oxide semiconductor layer. In order not to lowerthe crystallinity of the oxide semiconductor layer, the oxidesemiconductor layer is controlled to have a region in which theconcentration of silicon is lower than 1×10¹⁹ atoms/cm³, preferablyhigher than or equal to 1×10¹⁸ atoms/cm³ and lower than 5×10¹⁸atoms/cm³. Furthermore, the oxide semiconductor layer is controlled tohave a region in which the concentration of carbon is lower than 1×10¹⁹atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, morepreferably higher than or equal to 6×10¹⁷ atoms/cm³ and lower than1×10¹⁸ atoms/cm³.

A transistor in which a highly purified oxide semiconductor film is usedfor a channel formation region exhibits extremely low off-state current.When voltage between a source and a drain is set at about 0.1 V, 5 V, or10 V, for example, the off-state current per channel width of thetransistor can be as low as several yoctoamperes per micrometer toseveral zeptoamperes per micrometer.

As the gate insulating film of the transistor, an insulating filmcontaining silicon is used in many cases; thus, it is preferable that,as in the transistor in one embodiment of the present invention, aregion of the oxide semiconductor layer that serves as a channel not bein contact with the gate insulating film for the above reason. In thecase where a channel is formed at the interface between the gateinsulating film and the oxide semiconductor layer, scattering ofcarriers occurs at the interface, so that the field-effect mobility ofthe transistor is reduced in some cases. Also from the view of theabove, it is preferable that the region of the oxide semiconductor layerthat serves as a channel be separated from the gate insulating film.

Accordingly, with the oxide semiconductor layer 130 having a layeredstructure including the oxide semiconductor layers 130 a to 130 c, achannel can be formed in the oxide semiconductor layer 130 b; thus, thetransistor can have high field-effect mobility and stable electricalcharacteristics.

In a band structure, the conduction band minimums of the oxidesemiconductor layers 130 a to 130 c are continuous. This can beunderstood also from the fact that the compositions of the oxidesemiconductor layers 130 a to 130 c are close to one another and oxygenis easily diffused among the oxide semiconductor layers 130 a to 130 c.Thus, the oxide semiconductor layers 130 a to 130 c have a continuousphysical property though they have different compositions and form astack. In the drawings, interfaces between the oxide semiconductorlayers of the stack are indicated by dotted lines.

The oxide semiconductor layer 130 in which layers containing the samemain components are stacked is formed to have not only a simple layeredstructure of the layers but also a continuous energy band (here, inparticular, a well structure having a U shape in which the conductionband minimums are continuous (U-shape well)). In other words, thelayered structure is formed such that there exists no impurity thatforms a defect level such as a trap center or a recombination center ateach interface. If impurities exist between the stacked oxidesemiconductor layers, the continuity of the energy band is lost andcarriers disappear by a trap or recombination at the interface.

For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:3:2, 1:3:3, 1:3:4, 1:3:6, 1:4:5, 1:6:4, or 1:9:6 can be used for theoxide semiconductor layers 130 a and 130 c, and an In—Ga—Zn oxide whoseatomic ratio of In to Ga and Zn is 1:1:1, 2:1:3, 5:5:6, or 3:1:2 can beused for the oxide semiconductor layer 130 b. In the case where each ofthe oxide semiconductor layers 130 a, 130 b, and 130 c is formed usingthe above oxide as a sputtering target, the atomic ratio of eachsputtering target and that of each formed oxide semiconductor layer arenot necessarily consistent with each other.

The oxide semiconductor layer 130 b of the oxide semiconductor layer 130serves as a well, so that a channel is formed in the oxide semiconductorlayer 130 b. Since the conduction band minimums are continuous, theoxide semiconductor layer 130 can also be referred to as a U-shapedwell. Furthermore, a channel formed to have such a structure can also bereferred to as a buried channel.

Trap levels due to impurities or defects might be formed in the vicinityof the interface between an insulating layer such as a silicon oxidefilm and each of the oxide semiconductor layers 130 a and 130 c. Theoxide semiconductor layer 130 b can be distanced away from the traplevels owing to existence of the oxide semiconductor layers 130 a and130 c.

However, when the energy differences between the conduction band minimumof the oxide semiconductor layer 130 b and the conduction band minimumof each of the oxide semiconductor layers 130 a and 130 c are small, anelectron in the oxide semiconductor layer 130 b might reach the traplevel by passing over the energy differences. When the electron istrapped in the trap level, negative charge is generated at the interfacewith the insulating layer, so that the threshold voltage of thetransistor is shifted in a positive direction.

The oxide semiconductor layers 130 a to 130 c preferably include crystalparts. In particular, when crystals with c-axis alignment are used, thetransistor can have stable electrical characteristics. Moreover,crystals with c-axis alignment are resistant to bending; therefore,using such crystals can improve the reliability of a semiconductordevice using a flexible substrate.

As the conductive layer 140 functioning as a source electrode layer andthe conductive layer 150 functioning as a drain electrode layer, forexample, a single layer or a stacked layer formed using a materialselected from Al, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, and Sc and alloysof any of these metal materials can be used. Typically, it is preferableto use Ti, which is particularly easily bonded to oxygen, or W, whichhas a high melting point and thus makes subsequent process temperaturescomparatively high. It is also possible to use a stack of any of theabove materials and Cu or an alloy such as Cu—Mn, which has lowresistance. In the transistors 105, 106, 111, and 112, for example, itis possible to use W for the conductive layers 141 and 151 and use astack of Ti and Al for the conductive layers 142 and 152.

The above materials are capable of extracting oxygen from an oxidesemiconductor film. Therefore, in a region of the oxide semiconductorfilm that is in contact with any of the above materials, oxygen isreleased from the oxide semiconductor layer and an oxygen vacancy isformed. Hydrogen slightly contained in the film and the oxygen vacancyare bonded to each other, so that the region is markedly changed to ann-type region. Accordingly, the n-type region can serve as a source or adrain of the transistor.

In the case where W is used for the conductive layers 140 and 150, theconductive layers 140 and 150 may be doped with nitrogen. Doping withnitrogen can appropriately lower the capability of extracting oxygen andprevent the n-type region from spreading to a channel region. It ispossible to prevent the n-type region from spreading to a channel regionalso by using a stack of W and an n-type semiconductor layer as theconductive layers 140 and 150 and putting the n-type semiconductor layerin contact with the oxide semiconductor layer. As the n-typesemiconductor layer, an In—Ga—Zn oxide, zinc oxide, indium oxide, tinoxide, indium tin oxide, or the like to which nitrogen is added can beused.

The insulating layer 160 functioning as a gate insulating film can beformed using an insulating film containing one or more of aluminumoxide, magnesium oxide, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,and tantalum oxide. The insulating layer 160 may be a stack includingany of the above materials. The insulating layer 160 may contain La, N,Zr, or the like as an impurity.

An example of a layered structure of the insulating layer 160 isdescribed. The insulating layer 160 includes, for example, oxygen,nitrogen, silicon, or hafnium. Specifically, the insulating layer 160preferably includes hafnium oxide and silicon oxide or siliconoxynitride.

Hafnium oxide and aluminum oxide have higher dielectric constants thansilicon oxide and silicon oxynitride. Therefore, the insulating layer160 using hafnium oxide or aluminum oxide can have larger thickness thanthe insulating layer 160 using silicon oxide, so that leakage currentdue to tunnel current can be reduced. That is, a transistor with lowoff-state current can be provided. Moreover, hafnium oxide with acrystalline structure has a higher dielectric constant than hafniumoxide with an amorphous structure. Therefore, it is preferable to usehafnium oxide with a crystalline structure in order to provide atransistor with low off-state current. Examples of the crystal structureinclude a monoclinic crystal structure and a cubic crystal structure.Note that one embodiment of the present invention is not limited to theabove examples.

For the insulating layers 120 and 160 in contact with the oxidesemiconductor layer 130, a film that releases less nitrogen oxide ispreferably used. In the case where the oxide semiconductor is in contactwith an insulating layer that releases a large amount of nitrogen oxide,the density of states due to nitrogen oxide becomes high in some cases.For the insulating layers 120 and 160, for example, an oxide insulatinglayer such as a silicon oxynitride film or an aluminum oxynitride filmthat releases less nitrogen oxide can be used.

A silicon oxynitride film that releases less nitrogen oxide is a film ofwhich the amount of released ammonia is larger than the amount ofreleased nitrogen oxide in TDS; the amount of released ammonia istypically greater than or equal to 1×10¹⁸ molecules/cm³ and less than orequal to 5×10¹⁹ molecules/cm³. Note that the amount of released ammoniais the amount of ammonia released by heat treatment with which thesurface temperature of the film becomes higher than or equal to 50° C.and lower than or equal to 650° C., preferably higher than or equal to50° C. and lower than or equal to 550° C.

By using the above oxide insulating layer for the insulating layers 120and 160, a shift in the threshold voltage of the transistor can bereduced, which leads to reduced fluctuations in the electricalcharacteristics of the transistor.

For the conductive layer 170 functioning as a gate electrode layer, forexample, a conductive film formed using Al, Ti, Cr, Co, Ni, Cu, Y, Zr,Mo, Ru, Ag, Mn, Nd, Sc, Ta, W, or the like can be used. Alternatively,an alloy or a conductive nitride of any of these materials may be used.Alternatively, a stack of a plurality of materials selected from thesematerials, alloys of these materials, and conductive nitrides of thesematerials may be used. Typically, tungsten, a stack of tungsten andtitanium nitride, a stack of tungsten and tantalum nitride, or the likecan be used. Alternatively, Cu or an alloy such as Cu—Mn, which has lowresistance, or a stack of any of the above materials and Cu or an alloysuch as Cu—Mn may be used. In this embodiment, tantalum nitride is usedfor the conductive layer 171 and tungsten is used for the conductivelayer 172 to form the conductive layer 170.

As the insulating layer 175, a silicon nitride film, an aluminum nitridefilm, or the like containing hydrogen can be used. In the transistors103, 104, 106, 109, 110, and 112 described in Embodiment 2, when aninsulating film containing hydrogen is used as the insulating layer 175,part of the oxide semiconductor layer can have n-type conductivity. Inaddition, a nitride insulating film functions as a blocking film againstmoisture and the like and can improve the reliability of the transistor.

An aluminum oxide film can also be used as the insulating layer 175. Itis particularly preferable to use an aluminum oxide film as theinsulating layer 175 in the transistors 101, 102, 105, 107, 108, and 111described in Embodiment 2. The aluminum oxide film has a high blockingeffect of preventing penetration of both oxygen and impurities such ashydrogen and moisture. Accordingly, during and after the manufacturingprocess of the transistor, the aluminum oxide film can suitably functionas a protective film that has effects of preventing entry of impuritiessuch as hydrogen and moisture into the oxide semiconductor layer 130,preventing release of oxygen from the oxide semiconductor layer, andpreventing unnecessary release of oxygen from the insulating layer 120.Furthermore, oxygen contained in the aluminum oxide film can be diffusedinto the oxide semiconductor layer.

Furthermore, the insulating layer 180 is preferably formed over theinsulating layer 175. The insulating layer 180 can be formed using aninsulating film containing one or more of magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating layer 180 may be a stack of any of the above materials.

Here, like the insulating layer 120, the insulating layer 180 preferablycontains oxygen more than that in the stoichiometric composition. Oxygenreleased from the insulating layer 180 can be diffused into the channelformation region in the oxide semiconductor layer 130 through theinsulating layer 160, so that oxygen vacancies formed in the channelformation region can be filled with oxygen. In this manner, stableelectrical characteristics of the transistor can be achieved.

High integration of a semiconductor device requires miniaturization of atransistor. However, it is known that miniaturization of a transistorcauses deterioration of electrical characteristics of the transistor. Inparticular, a decrease in channel width causes a reduction in on-statecurrent.

In the transistors 107 to 112 in one embodiment of the presentinvention, the oxide semiconductor layer 130 c is formed to cover theoxide semiconductor layer 130 b where a channel is formed; thus, achannel formation layer is not in contact with the gate insulating film.Accordingly, scattering of carriers at the interface between the channelformation layer and the gate insulating film can be reduced and theon-state current of the transistor can be increased.

In the transistor in one embodiment of the present invention, asdescribed above, the gate electrode layer (the conductive layer 170) isformed to electrically surround the oxide semiconductor layer 130 in thechannel width direction; accordingly, a gate electric field is appliedto the oxide semiconductor layer 130 in a direction perpendicular to itsside surface in addition to a direction perpendicular to its topsurface. In other words, a gate electric field is applied to the entirechannel formation layer and effective channel width is increased,leading to a further increase in the on-state current.

Furthermore, in the transistor in one embodiment of the presentinvention in which the oxide semiconductor layer 130 has a two-layerstructure or a three-layer structure, since the oxide semiconductorlayer 130 b where a channel is formed is provided over the oxidesemiconductor layer 130 a, an effect of making an interface state lesslikely to be formed is obtained. In the transistor in one embodiment ofthe present invention in which the oxide semiconductor layer 130 has athree-layer structure, since the oxide semiconductor layer 130 b ispositioned at the middle of the three-layer structure, an effect ofeliminating the influence of an impurity that enters from upper andlower layers on the oxide semiconductor layer 130 b is obtained as well.Therefore, the transistor can achieve not only the increase in theon-state current of the transistor but also stabilization of thethreshold voltage and a reduction in the S value (subthreshold value).Thus, current when gate voltage VG is 0 V can be reduced and powerconsumption can be reduced. In addition, since the threshold voltage ofthe transistor becomes stable, long-term reliability of thesemiconductor device can be improved. Furthermore, the transistor in oneembodiment of the present invention is suitable for a highly integratedsemiconductor device because deterioration of electrical characteristicsdue to miniaturization is reduced.

Although the variety of films such as the metal films, the semiconductorfilms, and the inorganic insulating films that are described in thisembodiment typically can be formed by sputtering or plasma-enhanced CVD,such films may be formed by another method such as thermal CVD. Examplesof thermal CVD include metal organic chemical vapor deposition (MOCVD)and atomic layer deposition (ALD).

Since plasma is not used for deposition, thermal CVD has an advantagethat no defect due to plasma damage is generated.

Deposition by thermal CVD may be performed in such a manner that asource gas and an oxidizer are supplied to the chamber at the same time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is caused in the vicinity of thesubstrate or over the substrate.

Deposition by ALD is performed in such a manner that the pressure in achamber is set to an atmospheric pressure or a reduced pressure, sourcegases for reaction are introduced into the chamber and reacted, and thenthe sequence of gas introduction is repeated. An inert gas (e.g., argonor nitrogen) may be introduced as a carrier gas with the source gases.

For example, two or more kinds of source gases may be sequentiallysupplied to the chamber. In that case, after reaction of a first sourcegas, an inert gas is introduced, and then a second source gas isintroduced so that the source gases are not mixed. Alternatively, thefirst source gas may be exhausted by vacuum evacuation instead ofintroduction of the inert gas, and then the second source gas may beintroduced. The first source gas is adsorbed on the surface of thesubstrate and reacted to form a first layer, and then, the second sourcegas introduced is absorbed and reacted. As a result, a second layer isstacked over the first layer, so that a thin film is formed. Thesequence of gas introduction is controlled and repeated more than onceuntil desired thickness is obtained, so that a thin film with excellentstep coverage can be formed. The thickness of the thin film can beadjusted by the number of repetition times of the sequence of gasintroduction; therefore, ALD makes it possible to accurately adjustthickness and thus is suitable for manufacturing a minute FET.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film that have been disclosed in theembodiments can be formed by thermal CVD such as MOCVD or ALD. Forexample, in the case where an In—Ga—Zn—O film is formed, trimethylindium(In(CH₃)₃), trimethylgallium (Ga(CH₃)₃), and dimethylzinc (Zn(CH₃)₂) canbe used. Without limitation to the above combination, triethylgallium(Ga(C₂H₅)₃) can be used instead of trimethylgallium and diethylzinc(Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by adeposition apparatus using ALD, two kinds of gases, i.e., ozone (O₃) asan oxidizer and a source material gas which is obtained by vaporizingliquid containing a solvent and a hafnium precursor (hafnium alkoxideand a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH,Hf[N(CH₃)₂]₄) and tetrakis(ethylmethylamide)hafnium) are used.

For example, in the case where an aluminum oxide film is formed by adeposition apparatus using ALD, two kinds of gases, i.e., H₂O as anoxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor (e.g., trimethylaluminum(TMA, Al(CH₃)₃)) are used. Examples of another material includetris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using ALD, hexachlorodisilane is adsorbed on asurface where a film is to be formed, and radicals of an oxidizing gas(e.g., O₂ or dinitrogen monoxide) are supplied to react with anadsorbate.

For example, in the case where a tungsten film is formed by a depositionapparatus using ALD, a WF₆ gas and a B₂H₆ gas are sequentiallyintroduced to form an initial tungsten film, and then a WF₆ gas and anH₂ gas are sequentially introduced to form a tungsten film. Note that anSiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed by a deposition apparatus using ALD, anIn(CH₃)₃ gas and an O₃ gas) are sequentially introduced to form an In—Olayer, a Ga(CH₃)₃ gas and an O₃ gas) are sequentially introduced to forma Ga—O layer, and then a Zn(CH₃)₂ gas and an O₃ gas) are sequentiallyintroduced to form a Zn—O layer. Note that the order of these layers isnot limited to this example. A mixed compound layer such as an In—Ga—Olayer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by using thesegases. Although an H₂O gas which is obtained by bubbling with an inertgas such as Ar may be used instead of an O₃ gas), it is preferable touse an O₃ gas), which does not contain H.

A facing-target-type sputtering apparatus can be used for deposition ofan oxide semiconductor layer. Deposition using the facing-target-typesputtering apparatus can also be referred to as vapor deposition SP(VDSP).

When an oxide semiconductor layer is deposited using afacing-target-type sputtering apparatus, plasma damage to the oxidesemiconductor layer at the time of deposition can be reduced. Thus,oxygen vacancies in a film can be reduced. In addition, the use of thefacing-target-type sputtering apparatus enables low-pressure deposition.Accordingly, the concentration of impurities (e.g., hydrogen, a rare gas(e.g., argon), or water) in a deposited oxide semiconductor layer can belowered.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 4

The structure of an oxide semiconductor film that can be used for oneembodiment of the present invention is described below.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Theterm “perpendicular” indicates that an angle formed between two straightlines is greater than or equal to 80° and less than or equal to 100°,and accordingly includes the case where the angle is greater than orequal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

An oxide semiconductor film is roughly classified into anon-single-crystal oxide semiconductor film and a single-crystal oxidesemiconductor film. The non-single-crystal oxide semiconductor filmmeans any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS)film, a polycrystalline oxide semiconductor film, a microcrystallineoxide semiconductor film, an amorphous oxide semiconductor film, and thelike.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image(also referred to as a high-resolution TEM image) of a bright-fieldimage and a diffraction pattern of the CAAC-OS film is observed.Consequently, a plurality of crystal parts are observed clearly.However, in the high-resolution TEM image, a boundary between crystalparts, i.e., a grain boundary is not observed clearly. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface (cross-sectional TEM image), metal atoms are arranged in alayered manner in the crystal parts. Each metal atom layer has amorphology that reflects a surface over which the CAAC-OS film is formed(also referred to as a formation surface) or a top surface of theCAAC-OS film, and is provided parallel to the formation surface or thetop surface of the CAAC-OS film.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface (planar TEM image), metal atoms are arranged in atriangular or hexagonal configuration in the crystal parts. However,there is no regularity of arrangement of metal atoms between differentcrystal parts.

The CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic order of theoxide semiconductor film by depriving the oxide semiconductor film ofoxygen and causes a decrease in crystallinity. Furthermore, a heavymetal such as iron or nickel, argon, carbon dioxide, or the like has alarge atomic radius (molecular radius), and thus disturbs the atomicorder of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas “highly purified intrinsic” or “substantially highly purifiedintrinsic.” A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor film has few carrier generation sources,and thus can have low carrier density. Thus, a transistor including theoxide semiconductor film rarely has negative threshold voltage (israrely normally on). The highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has few carriertraps. Accordingly, the transistor including the oxide semiconductorfilm has few variations in electrical characteristics and highreliability. Charge trapped by the carrier traps in the oxidesemiconductor film takes a long time to be released and may behave likefixed charge. Thus, the transistor that includes the oxide semiconductorfilm having high impurity concentration and high density of defectstates has unstable electrical characteristics in some cases.

In a transistor including the CAAC-OS film, changes in electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light are small.

Next, a microcrystalline oxide semiconductor film is described.

A microcrystalline oxide semiconductor film has a region where a crystalpart is observed in a high-resolution TEM image and a region where acrystal part is not clearly observed in a high-resolution TEM image. Inmost cases, a crystal part in the microcrystalline oxide semiconductorfilm is greater than or equal to 1 nm and less than or equal to 100 nm,or greater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as ananocrystalline oxide semiconductor (nc-OS) film. In a high-resolutionTEM image, a grain boundary cannot be found clearly in the nc-OS film insome cases.

In the nc-OS film, a microscopic region (e.g., a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has periodic atomic order. There is no regularityof crystal orientation between different crystal parts in the nc-OSfilm. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak that shows a crystalplane does not appear. Furthermore, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter larger than the diameterof a crystal part (e.g., larger than or equal to 50 nm). Meanwhile,spots are shown in a nanobeam electron diffraction pattern of the nc-OSfilm obtained by using an electron beam having a probe diameter close toor smaller than the diameter of a crystal part. Furthermore, in ananobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are observed in some cases.Also in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of spots are shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularitythan an amorphous oxide semiconductor film. Thus, the nc-OS film has alower density of defect states than the amorphous oxide semiconductorfilm. Note that there is no regularity of crystal orientation betweendifferent crystal parts in the nc-OS film; thus, the nc-OS film has ahigher density of defect states than the CAAC-OS film.

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state as in quartz.

In a high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Furthermore, a halo pattern is shown but a spot is not shown in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(amorphous-like OS) film.

In a high-resolution TEM image of the amorphous-like OS film, a void maybe seen. Furthermore, in the high-resolution TEM image, there are aregion where a crystal part is clearly observed and a region where acrystal part is not observed. In the amorphous-like OS film,crystallization by a slight amount of electron beam used for TEMobservation occurs and growth of the crystal part is found sometimes. Incontrast, crystallization by a slight amount of electron beam used forTEM observation is less observed in the nc-OS film having good quality.

Note that the crystal part size in the amorphous-like OS film and thenc-OS film can be measured using high-resolution TEM images. Forexample, an InGaZnO₄ crystal has a layered structure in which twoGa—Zn—O layers are included between In—O layers. A unit cell of theInGaZnO₄ crystal has a structure in which nine layers of three In—Olayers and six Ga—Zn—O layers are layered in the c-axis direction.Accordingly, the spacing between these adjacent layers is equivalent tothe lattice spacing on the (009) plane (also referred to as a d value).The value is calculated to be 0.29 nm from crystal structure analysis.Thus, each of the lattice fringes in which the spacing therebetween isfrom 0.28 nm to 0.30 nm corresponds to the a-b plane of the InGaZnO₄crystal, focusing on the lattice fringes in the high-resolution TEMimage.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, anamorphous-like OS film, a microcrystalline oxide semiconductor film, anda CAAC-OS film, for example.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 5

An imaging device in one embodiment of the present invention and asemiconductor device including the imaging device can be used fordisplay devices, personal computers, or image reproducing devicesprovided with recording media (typically, devices that reproduce thecontent of recording media such as digital versatile discs (DVD) andhave displays for displaying the reproduced images). Furthermore, aselectronic devices that can include the imaging device in one embodimentof the present invention and the semiconductor device including theimaging device, cellular phones, game machines (including portable gamemachines), portable information terminals, e-book readers, cameras suchas video cameras and digital still cameras, goggle-type displays (headmounted displays), navigation systems, audio reproducing devices (e.g.,car audio systems and digital audio players), copiers, facsimiles,printers, multifunction printers, automated teller machines (ATM),vending machines, and the like can be given. FIGS. 34A to 34F illustratespecific examples of these electronic devices.

FIG. 34A illustrates a portable game machine, which includes housings901 and 902, display portions 903 and 904, a microphone 905, speakers906, an operation key 907, a stylus 908, a camera 909, and the like.Although the portable game machine in FIG. 34A includes the two displayportions 903 and 904, the number of display portions included in theportable game machine is not limited to this. The imaging device in oneembodiment of the present invention can be used for the camera 909.

FIG. 34B illustrates a portable data terminal, which includes a firsthousing 911, a display portion 912, a camera 919, and the like. Thetouch panel function of the display portion 912 enables input and outputof information. The imaging device in one embodiment of the presentinvention can be used for the camera 919.

FIG. 34C illustrates a wrist-watch-type information terminal, whichincludes a housing 931, a display portion 932, a wristband 933, a camera939, and the like. The display portion 932 may be a touch panel. Theimaging device in one embodiment of the present invention can be usedfor the camera 939.

FIG. 34D illustrates a cellular phone, which includes a display portion952, a microphone 957, a speaker 954, a camera 959, an input/outputterminal 956, an operation button 955, and the like in a housing 951.The imaging device in one embodiment of the present invention can beused for the camera 959.

FIG. 34E illustrates a digital camera, which includes a housing 961, ashutter button 962, a microphone 963, a light-emitting portion 967, alens 965, and the like. The imaging device in one embodiment of thepresent invention can be used provided in a focus of the lens 965.

FIG. 34F illustrates a video camera, which includes a first housing 971,a second housing 972, a display portion 973, operation keys 974, a lens975, a joint 976, and the like. The operation keys 974 and the lens 975are provided in the first housing 971, and the display portion 973 isprovided in the second housing 972. The first housing 971 and the secondhousing 972 are connected to each other with the joint 976, and an anglebetween the first housing 971 and the second housing 972 can be changedwith the joint 976. An image displayed on the display portion 973 may beswitched in accordance with the angle between the first housing 971 andthe second housing 972 at the joint 976. The imaging device in oneembodiment of the present invention can be provided in a focus of thelens 975.

This embodiment can be combined with any of the other embodimentsdescribed in this specification as appropriate.

This application is based on Japanese Patent Application serial No.2015-016743 filed with Japan Patent Office on Jan. 30, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An electronic device comprising: a lens; aphotoelectric conversion element; a first transistor; a secondtransistor; a first wiring; and a second wiring, wherein the lens andthe photoelectric conversion element overlap each other, wherein oneelectrode of the photoelectric conversion element is electricallyconnected to the first wiring, wherein the other electrode of thephotoelectric conversion element is electrically connected to one of asource electrode and a drain electrode of the first transistor, whereinthe other of the source electrode and the drain electrode of the firsttransistor is electrically connected to a gate electrode of the secondtransistor, wherein a gate electrode of the first transistor iselectrically connected to the second wiring, and wherein the highestpotential supplied to the second wiring is lower than a potentialsupplied to the first wiring.
 2. The electronic device according toclaim 1, wherein light penetrating the lens reaches the photoelectricconversion element.
 3. The electronic device according to claim 1,wherein a channel formation region of the first transistor comprises anoxide semiconductor.
 4. The electronic device according to claim 3,wherein the oxide semiconductor includes In, Zn, and Ga.
 5. Theelectronic device according to claim 1, wherein each of a channelformation region of the first transistor and a channel formation regionof the second transistor comprises silicon.
 6. An electronic devicecomprising: a housing; a display portion provided in the housing; and acamera provided in the housing, the camera comprising: a lens; aphotoelectric conversion element; a first transistor; a secondtransistor; a first wiring; and a second wiring, wherein the lens andthe photoelectric conversion element overlap each other, wherein oneelectrode of the photoelectric conversion element is electricallyconnected to the first wiring, wherein the other electrode of thephotoelectric conversion element is electrically connected to one of asource electrode and a drain electrode of the first transistor, whereinthe other of the source electrode and the drain electrode of the firsttransistor is electrically connected to a gate electrode of the secondtransistor, wherein a gate electrode of the first transistor iselectrically connected to the second wiring, wherein the highestpotential supplied to the second wiring is lower than a potentialsupplied to the first wiring, and wherein the camera and the displayportion are provided on the same side of the electronic device.
 7. Theelectronic device according to claim 6, wherein light penetrating thelens reaches the photoelectric conversion element.
 8. The electronicdevice according to claim 6, wherein a channel formation region of thefirst transistor comprises an oxide semiconductor.
 9. The electronicdevice according to claim 8, wherein the oxide semiconductor includesIn, Zn, and Ga.
 10. The electronic device according to claim 6, whereineach of a channel formation region of the first transistor and a channelformation region of the second transistor comprises silicon.
 11. Animaging device comprising: a photoelectric conversion element; a firsttransistor; a second transistor; a first wiring; and a second wiring,wherein one electrode of the photoelectric conversion element iselectrically connected to the first wiring, wherein the other electrodeof the photoelectric conversion element is electrically connected to oneof a source electrode and a drain electrode of the first transistor,wherein the other of the source electrode and the drain electrode of thefirst transistor is electrically connected to a gate electrode of thesecond transistor, wherein a gate electrode of the first transistor iselectrically connected to the second wiring, and wherein the highestpotential supplied to the second wiring is lower than a potentialsupplied to the first wiring.
 12. The imaging device according to claim11, wherein a channel formation region of the first transistor comprisesan oxide semiconductor.
 13. The imaging device according to claim 12,wherein the oxide semiconductor includes In, Zn, and Ga.
 14. The imagingdevice according to claim 11, wherein each of a channel formation regionof the first transistor and a channel formation region of the secondtransistor comprises silicon.